TypeError: Argument 1 passed to Phalcon\Text::lower() must be of the type string, null given

<div class="page-bg chord-bg" style="background-image: url('<?= $this->url->get(($chord->Artist->artwork != null ? $chord->Artist->artwork : 'images/bg_default.jpg')) ?>');background-position: top"></div>
<div class="row-col">
  <?= $this->partial('components/left-sidebar') ?>
  <div class="col-lg-9 b-r no-border-md">
    <div class="padding b-b">
      <nav aria-label="breadcrumb">
        <ol class="breadcrumb text-sm">
          <li class="breadcrumb-item"><a href="https://www.musikord.com/">Home</a></li>
          <li class="breadcrumb-item"><a href="#"><?= $this->tag->linkTo(['artists/' . substr(Phalcon\Text::lower($chord->Artist->slug), 0, 1) . '/' . (Phalcon\Text::lower($chord->Artist->slug)) . '.html', $chord->Artist->title]) ?></a></li>
          <li style="width: max-content" class="breadcrumb-item active text-ellipsis" aria-current="page"><?= $chord->title ?></li>
        </ol>
      </nav>
      <div class="row-col">
        <div class="col-sm w w-auto-xs m-b">
          <div class="item w r" data-id="item-<?= $chord->id ?>" data-src="https://www.youtube.com/watch?v=<?= $chord->video ?>">
            <div class="item-media semi-rounded hidden-xs-down">
              <div class="item-media-content" style="background-image: url('<?= $this->url->get(($chord->Artist->artwork != null ? $chord->Artist->artwork : 'images/bg_default.jpg')) ?>')" alt="<?= $chord->title ?>">
                <div style="display: none" class="item-overlay center mejs-button mejs-playpause-button">
                  <button class="btn-playpause" aria-controls="mep_0"></button>
                </div>
              </div>
              <div class="item-info" style="display: none">
                <div class="item-title text-allipsis">
                  <?= $this->tag->linkTo([$_SERVER['REQUEST_URI'], $chord->title]) ?>
                </div>
                <div class="item-author text-sm text-ellipsis">
                  <?= $this->tag->linkTo(['artist/' . Phalcon\Text::lower($chord->Artist->slug)]) ?>
                </div>
              </div>
            </div>
 
            <?php if ($this->length($chord->Artist->wallpaper) > 0) { ?>
            <span style="position: absolute; right: 5px; bottom: 5px;">
              <?= $this->tag->image([$chord->Artist->artwork, 'style' => 'border:4px solid white;width: 50px;box-shadow: 0 4px 8px 0 rgba(0, 0, 0, 0.2), 0 4px 8px 0 rgba(0, 0, 0, 0.19);', 'class' => 'avatar-w-60']) ?>
            </span>
            <?php } ?>
 
          </div>
        </div>
        <div class="col-sm">
          <div class="p-l-md no-padding-xs">
            <div class="page-title">
              <h1 class="inline title-mobile" alt="<?= $chord->title ?>" title="<?= $chord->title ?>">
                <?= $chord->title ?>
              </h1>
              <?= $this->partial('components/explicit') ?>
            </div>
 
            <p class="item-author text-ellipsis text-muted"><span class="text-muted">by</span>
              <?= $this->tag->linkTo(['artists/' . substr(Phalcon\Text::lower($chord->Artist->slug), 0, 1) . '/' . (Phalcon\Text::lower($chord->Artist->slug)) . '.html', $chord->Artist->title]) ?>
              <?php if ($this->length($featuring) > 0) { ?>
              <span class="text-muted">feat</span> <?php $v166114941458396728911iterator = $featuring; $v166114941458396728911incr = 0; $v166114941458396728911loop = new stdClass(); $v166114941458396728911loop->self = &$v166114941458396728911loop; $v166114941458396728911loop->length = count($v166114941458396728911iterator); $v166114941458396728911loop->index = 1; $v166114941458396728911loop->index0 = 1; $v166114941458396728911loop->revindex = $v166114941458396728911loop->length; $v166114941458396728911loop->revindex0 = $v166114941458396728911loop->length - 1; ?><?php foreach ($v166114941458396728911iterator as $featured_artist) { ?><?php $v166114941458396728911loop->first = ($v166114941458396728911incr == 0); $v166114941458396728911loop->index = $v166114941458396728911incr + 1; $v166114941458396728911loop->index0 = $v166114941458396728911incr; $v166114941458396728911loop->revindex = $v166114941458396728911loop->length - $v166114941458396728911incr; $v166114941458396728911loop->revindex0 = $v166114941458396728911loop->length - ($v166114941458396728911incr + 1); $v166114941458396728911loop->last = ($v166114941458396728911incr == ($v166114941458396728911loop->length - 1)); ?>
              <?= $this->tag->linkTo(['artists/' . substr(Phalcon\Text::lower($featured_artist->slug), 0, 1) . '/' . (Phalcon\Text::lower($featured_artist->slug)) . '.html', $featured_artist->title]) ?><?= (!$v166114941458396728911loop->last ? ', ' : null) ?>
              <?php $v166114941458396728911incr++; } ?>
              <?php } ?>
            </p>
 
            <div class="item-action m-b">
              <a href="#" id="share-btn-2" class="btn btn-icon white rounded btn-share pull-right" title="Share this page">
                <i class="fa fa-share-alt"></i>
              </a>
 
              <button style="display: none" class="btn-playpause text-primary m-r-sm"></button>
              <a href="#" style="display:none" class="btn btn-icon rounded btn-favorite" data-toggle="modal" data-target="#modal-scrolling" title="Expand chord dan lirik">
                <i class="fa fa-expand" aria-hidden="true"></i>
              </a>
              <a href="#" class="btn btn-icon btn-xs rounded btn-favorite" aria-label="View">
                <i class="fa fa-eye"></i>
              </a>
              <span class="text-muted">
                <span dataconvert>
                  <?= $chord->view ?>
                </span>
              </span>
              <!--
              <a href="#" class="btn btn-icon rounded btn-favorite" data-toggle="modal" data-target="#list-modal" title="Lihat semua lagu" aria-label="Modal">
                <i class="fa fa-list-ul" aria-hidden="true"></i>
              </a>
              -->
              
 
              <span style="display: inline-block;margin-left: 5px;">
                <div class="sh-section__btn-options sh-btn-icon">
                  <a href="<?= $this->url->get('add-favourite/' . $chord->id) ?>" class="btn btn-xs btn-blue btn-outline rounded <?= (!$is_favourite ? 'blue' : 'red') ?>">
                    <i class="fa fa-bookmark" aria-hidden="true"></i> &nbsp;<?= (!$is_favourite ? 'Favorite' : 'Remove Favorite') ?>
                  </a>
                  <?php if (!$this->session->has('user')) { ?>
                  <b style="margin-left: -20px" class="label rounded danger up">New</b>
                  <?php } ?>
                </div>
              </span>
 
              <div class="inline dropdown m-l-xs">
                <a href="#" class="btn btn-icon rounded btn-more" data-toggle="dropdown" aria-label="Dropdown">
                  <i class="fa fa-ellipsis-h"></i>
                </a>
                <div class="dropdown-menu dropdown-menu-scale pull-right">
                  <?php if ($chord->itunes != null) { ?>
                  <a class="dropdown-item" href="<?= $chord->itunes ?>" target="_blank">
                    <i class="fa fa-apple" aria-hidden="true"></i> &nbsp;Buy on iTunes
                  </a>
                  <?php } ?>
                  <a class="dropdown-item" href="<?= $this->url->get('submit/chord') ?>">
                    <span class="material-icons">add</span> &nbsp;Submit Chords
                  </a>
                  <a class="dropdown-item" href="#" data-toggle="modal" data-target="#share-modal">
                    <i class="fa fa-share-alt" aria-hidden="true"></i> &nbsp;Share
                  </a>
                  <div class="dropdown-divider"></div>
                  <a class="dropdown-item" href="#" data-toggle="modal" data-target="#list-modal">
                    <i class="fa fa-list-ul" aria-hidden="true"></i> &nbsp;All Songs
                  </a>
                </div>
              </div>
            </div>
            <?php if ($chord->soundcloud != null) { ?>
            <span style="display: none" class='progress'>
              <div class='progress-bar'></div>
            </span>
            <?php } ?>
            <div class="item-meta">
              <?php $v166114941458396728911iterator = $chord->Artist->genre; $v166114941458396728911incr = 0; $v166114941458396728911loop = new stdClass(); $v166114941458396728911loop->self = &$v166114941458396728911loop; $v166114941458396728911loop->length = count($v166114941458396728911iterator); $v166114941458396728911loop->index = 1; $v166114941458396728911loop->index0 = 1; $v166114941458396728911loop->revindex = $v166114941458396728911loop->length; $v166114941458396728911loop->revindex0 = $v166114941458396728911loop->length - 1; ?><?php foreach ($v166114941458396728911iterator as $genre) { ?><?php $v166114941458396728911loop->first = ($v166114941458396728911incr == 0); $v166114941458396728911loop->index = $v166114941458396728911incr + 1; $v166114941458396728911loop->index0 = $v166114941458396728911incr; $v166114941458396728911loop->revindex = $v166114941458396728911loop->length - $v166114941458396728911incr; $v166114941458396728911loop->revindex0 = $v166114941458396728911loop->length - ($v166114941458396728911incr + 1); $v166114941458396728911loop->last = ($v166114941458396728911incr == ($v166114941458396728911loop->length - 1)); ?>
              <a style="text-transform: capitalize" href="<?= $this->url->get('artists/genre/' . urldecode($genre)) ?>" class="btn btn-xs rounded white">
                <?= urldecode($genre) ?>
              </a>
              <?php $v166114941458396728911incr++; } ?>
            </div>
          </div>
        </div>
      </div>
    </div>
 
    <div id="chord-section" class="padding <?= ($chord->chord != null ? 'col-lg-11' : 'col-lg-12') ?>">
 
      <?php if ($chord->chord != null) { ?>
      <div class="nav-active-border b-danger bottom m-b-md">
        <ul class="nav l-h-2x">
          <li class="nav-item m-r inline"> <a class="nav-link active" href="#chord" data-toggle="tab" data-target="#chord">Chords</a> </li>
          <li class="nav-item m-r inline"> <a class="nav-link" href="#lirik" data-toggle="tab" data-target="#lirik">Lyrics</a> </li>
          <li class="nav-item m-r inline"> <a class="nav-link" href="#tab1" data-toggle="tab" data-target="#tab1">Tab</a> </li>
          <li class="nav-item m-r inline"> <a class="nav-link" href="#video" data-toggle="tab" data-target="#video">Listen</a> </li>
          <?php if ($chord->itunes != null) { ?>
          <li class="nav-item m-r inline"> <a class="nav-link" href="#buy" data-toggle="tab" data-target="#buy"><i class="fa fa-apple" aria-hidden="true"></i> Buy</a> </li>
          <?php } ?>
        </ul>
      </div>
      <?php } ?>
      <div class="tab-content">
        <div class="tab-pane active" id="chord">
          <div class="row item-list item-list-md item-list-li" id="tracks">
            <div id="printableArea" class="col-md-12 col-lg-11">
              <?php if ($chord->chord != null) { ?>
              <div id="instrument-tabs" style="margin-bottom: 10px;">
                <button onclick="setInstrument('guitar')" class="active">Guitar</button>
                <button onclick="setInstrument('ukulele')">Ukulele</button>
                <button onclick="setInstrument('piano')">Piano</button>
              </div>
                <div id="chord-diagram-bar"></div>
              <div id="chord-wrapper">
                <pre class="chord-content" id="tab" style="background: transparent !important;width: 100%;"><?= $chord->chord ?></pre>
              </div>
              <?php } ?>
            </div>
          </div>
 
          <?php if ($chord->chord == null) { ?>
          <center class="m-t-lg text-center">
            <h3 class="hidden-xs-down">Chord not Available</h3>
            <h4 class="hidden-md-up">Chord not Available</h4>
            <p>Be the first to submit <span class="text-danger"><?= $chord->title ?></span>. Get more points to every chord you submit</p>
            <a style="margin-bottom:80px" href="<?= $this->url->get('submit/' . Phalcon\Text::lower($chord->slug)) ?>" class="btn btn-lg btn-outline danger rounded">
              <span class="material-icons">add</span> Submit chords
            </a>
          </center>
          <?php } ?>
        </div>
        <div class="tab-pane" id="lirik">
          <div class="row m-b">
            <div class="col-md-12 col-lg-9">
              <?php if ($chord->lyric != null) { ?>
              <pre class="chord" style="font-family: Helvetica;background: transparent !important;white-space: pre-line;font-weight: normal !important;padding: 0px" class="lyric-font">
              <?= $chord->lyric ?>
              </pre>
              <?php } else { ?>
              <div class="text-muted">Lyrics not available</div>
              <?php } ?>
            </div>
          </div>
        </div>
        <div class="tab-pane" id="tab1">
          <div class="row m-b">
            <div class="col-md-12 col-lg-9">
              <?php if ($chord->tab != null) { ?>
              <pre style="background: transparent !important;white-space: pre-line;" class="lyric-font">
              <?= $chord->tab ?>
              </pre>
              <?php } else { ?>
              <div class="text-muted">Tab not available</div>
              <?php } ?>
            </div>
          </div>
        </div>
        <div class="tab-pane" id="video">
          <div class="row">
 
            <div style="display:none;" class="col-xs-6 col-sm-6 col-md-4">
              <?= $this->partial('components/sponsor-card') ?>
            </div>
 
            <div class="col-xs-4 col-sm-4 col-md-4 col-lg-3">
              <div class="item r" data-id="item-3" data-src="https://api.soundcloud.com/tracks/79031167/stream?client_id=a10d44d431ad52868f1bce6d36f5234c">
                <div class="item-media semi-rounded">
                  <?php if ($chord->video != null) { ?>
                  <a href="#" data-theVideo="https://www.youtube.com/embed/<?= $chord->video ?>" data-toggle="modal" data-target="#videoModal" class="item-media-content" style="background-image: url('https://img.youtube.com/vi/<?= $chord->video ?>/0.jpg');width: 100%;height: 350px;object-fit: cover;background-position: 60% -50px;" alt="<?= $chord->title ?>" title="<?= $chord->title ?>" aria-label="Artwork"></a>
                  <?php } else { ?>
                  <a href="<?= $this->url->get('chord/' . $chord->slug) ?>" target="_blank" class="item-media-content" style="background-image: url('<?= $this->url->get('images/bg_default.jpg') ?>');" alt="<?= $chord->title ?>" title="<?= $chord->title ?>"></a>
                  <?php } ?>
                  <div class="item-overlay center">
                    <button href="#" data-theVideo="https://www.youtube.com/embed/<?= $chord->video ?>" data-toggle="modal" data-target="#videoModal" class="btn-playpause">Play</button>
                  </div>
                </div>
                <div class="item-info">
                  <div class="item-overlay bottom text-right">
                    <a href="#" class="btn-favorite">
                      <i class="fa fa-heart-o"></i>
                    </a>
                    <a href="#" class="btn-more" data-toggle="dropdown">
                      <i class="fa fa-ellipsis-h"></i>
                    </a>
                    <div class="dropdown-menu pull-right black lt"></div>
                  </div>
                  <div class="item-title text-ellipsis">
                    <a href="#" onclick="window.location.href='<?= $this->url->get('chord/' . $chord->slug) ?>'" href="<?= $this->url->get('chord/' . $chord->slug) ?>">
                      <?= $chord->title ?>
                    </a>
                  </div>
                  <div class="item-author text-sm text-ellipsis ">
                    <a href="#" onclick="window.location.href='<?= $this->url->get('artist/' . $chord->Artist->slug) ?>" href="<?= $this->url->get('artist/' . $chord->Artist->slug) ?>" class="text-muted">
                      <?= $chord->Artist->title ?>
                    </a>
                  </div>
                </div>
              </div>
            </div>
          </div>
        </div>
        <div class="tab-pane" id="buy">
          <div class="row m-b">
            <div class="col-md-12 col-lg-9">
              <a href="<?= $chord->itunes ?>" target="_blank">
                <i class="fa fa-apple" aria-hidden="true"></i> &nbsp;Buy song
                <span class="text-danger"><?= $chord->title ?> on iTunes store
              </a>
              <small>
                <i>
                  <p class="text-mute">Support artists with purchashing the original song!</p>
                </i>
              </small>
            </div>
          </div>
        </div>
        <?php if ($chord->RequestBy != null) { ?>
        <div class="list box box-shadow-z2">
          <div class="list-item">
            <div class="list-body">
              <span class="_500"><span class="text-muted"><i class="material-icons">account_circle</i> &nbsp;Chord request by </span>
                <?= $chord->RequestBy->name ?>
              </span>
              <small class="block text-muted">
                <?= $chord->RequestBy->bio ?>
              </small>
            </div>
          </div>
        </div>
        <?php } ?>
        <?php if ($chord->chord != null) { ?>
        <div class="see-more-container text-left m-t-md">
          <button id="share-btn-3" class="share-chord-button text-info">Share<span class="material-icons">arrow_outward</span></button>
          <button onclick="savePdf()" class="share-chord-button text-info">Save PDF<span style="font-size: 20px" class="material-icons">picture_as_pdf</span></button>
          <span class="share-chord-button pull-right text-muted">
            <?php if (!empty($chord->update_time)) { ?>
              <span style="margin-right: 3px;font-size: 20px" class="material-icons">access_time</span>Last updated: <?= $this->getDI()->get("timeAgoNoDay")->__invoke($chord->update_time) ?>
            <?php } ?>
          </span>
        </div>
        <?php } ?>
      </div>
      <!--
      <?php if (isset($current_artist)) { ?>
      <div class="hidden-sm-up p-t-md">
        <div sty id="tracks" class="row item-list item-list-xs item-list-li m-b no-padding-xs">
          <div class="col-lg-12 col-xs-12 no-padding-xs">
            <?php $v166114941458396728911iterator = $chord_list; $v166114941458396728911incr = 0; $v166114941458396728911loop = new stdClass(); $v166114941458396728911loop->self = &$v166114941458396728911loop; $v166114941458396728911loop->length = count($v166114941458396728911iterator); $v166114941458396728911loop->index = 1; $v166114941458396728911loop->index0 = 1; $v166114941458396728911loop->revindex = $v166114941458396728911loop->length; $v166114941458396728911loop->revindex0 = $v166114941458396728911loop->length - 1; ?><?php foreach ($v166114941458396728911iterator as $chord) { ?><?php $v166114941458396728911loop->first = ($v166114941458396728911incr == 0); $v166114941458396728911loop->index = $v166114941458396728911incr + 1; $v166114941458396728911loop->index0 = $v166114941458396728911incr; $v166114941458396728911loop->revindex = $v166114941458396728911loop->length - $v166114941458396728911incr; $v166114941458396728911loop->revindex0 = $v166114941458396728911loop->length - ($v166114941458396728911incr + 1); $v166114941458396728911loop->last = ($v166114941458396728911incr == ($v166114941458396728911loop->length - 1)); ?>
            <?= $this->callMacro('chord_list', [$chord]) ?>
            <?php $v166114941458396728911incr++; } ?>
          </div>
        </div>
      </div>
      <?php } ?>
      -->
      <div id="stopper"></div>
    </div>
    <div id="sticky-anchor"></div>
    <div class="padding col-lg-1 static-sticky hide-mobile" id="sticky">
      <div class="nav-active-border b-danger bottom m-b-md">
        <ul class="nav l-h-2x">
          <li class="nav-item inline pull-right button-transpose hide-mobile">
            <?php if ($chord->chord != null) { ?>
            <div class="scroll-control-container" id="scroll-controls-desktop">
              <!-- Left buttons -->
              <button onclick="transpose(1)" class="extra-btn btn circle btn-outline b-black rounded text-black btn-night"><span class="material-icons">plus_one</span></button>
              <button onclick="transpose(-1)" class="extra-btn btn circle btn-outline b-black rounded text-black btn-night"><span class="material-icons">exposure_neg_1</span></button>
              <button onclick="savePdf()" class="extra-btn btn circle btn-outline b-black rounded text-black btn-night"><span class="material-icons">picture_as_pdf</span></button>
            
              <!-- Center play button group -->
              <div id="center-controls-desktop" style="display: flex; flex-direction: column; align-items: center; gap: 0.6em;">
                <!-- add = top -->
                <!-- play = center -->
                <button id="play-btn-desktop" class="scroll-btn btn circle btn-outline black text-black btn-night-play">
                  <span class="material-icons">north</span>
                </button>
                <button id="speed-up-btn-desktop" class="scroll-btn hidden btn circle btn-outline black text-black btn-night-play">
                  <span class="material-icons">add</span>
                </button>
                <!-- remove = bottom -->
                <button id="speed-down-btn-desktop" class="scroll-btn hidden btn circle btn-outline black text-black btn-night-play">
                  <span class="material-icons">remove</span>
                </button>
              </div>
            
              <!-- Right buttons -->
              <button id="large" class="extra-btn btn circle btn-outline b-black rounded text-black btn-night"><span class="material-icons">text_increase</span></button>
              <button id="medium" class="extra-btn btn circle btn-outline b-black rounded text-black btn-night"><span class="material-icons">text_format</span></button>
              <button id="small" class="extra-btn btn circle btn-outline b-black rounded text-black btn-night"><span class="material-icons">text_decrease</span></button>
            </div>
            <?php } ?>
          </li>
        </ul>
      </div>
    </div>
    <?= $this->partial('components/footer-content', ['chord_id' => $chord->id]) ?>
  </div>
  <?= $this->partial('components/right-sidebar') ?>
</div>

<?php
define('BASE_PATH', dirname(__DIR__));
define('APP_PATH', BASE_PATH . '/app');
 
error_reporting(E_ALL);
ini_set('display_errors', 1);
 
require(BASE_PATH . '/vendor/autoload.php');
 
use Phalcon\Di\FactoryDefault;
use Phalcon\Debug;
 
// Enable Phalcon debug screen
$debug = new Debug();
$debug->listen();
 
try {
    $di = new FactoryDefault();
 
    include APP_PATH . '/config/router.php';
    include APP_PATH . '/config/services.php';
 
    $config = $di->getConfig();
    include APP_PATH . '/config/loader.php';
 
    $application = new \Phalcon\Mvc\Application($di);
    echo $application->handle($_SERVER['REQUEST_URI'])->getContent();
 
} catch (\Exception $e) {
    // Temporarily disable redirect for debugging
    // header('location:' . $config->application->baseUri . '404');
    // die($e->getMessage());
 
    echo '<pre>' . $e . '</pre>';
}