public function on404() {

    // 404 Fallback message.
    $response = '
    <div class="box">
      <div class="row">
        <div class="col-xs-3 col-sm-2 col-md-2 text-center mrgn-tp-md">
          <span class="glyphicon glyphicon-warning-sign glyphicon-error"></span>
        </div>
        <div class="col-xs-9 col-sm-10 col-md-10">
          <h2 class="mrgn-tp-md">' . $this->t('We couldn\'t find that Web page') . '</h2>
          <p class="pagetag"><strong>' . $this->t('Error 404') . '</strong></p>
        </div>
      </div>
      <p class="mrgn-tp-md">' . $this->t('We\'re sorry you ended up here. Sometimes a page gets moved or deleted.') . '</p>
    </div>';

    // Lookup our custom 404 content block.
    $block_id = $this->blockContentStorage->loadByProperties([
      'info' => '404',
      'type' => 'basic',
    ]);
    if (!empty($block_id)) {
      $response = $this->blockViewBuilder->view(reset($block_id));
    }

    return [
      '#type' => 'container',
      '#markup' => render($response),
      '#attributes' => [
        'class' => '404 error',
      ],
      '#weight' => 0,
    ];
  }