code: protected function renderCloseButton()
      {
          if (($closeButton = $this->closeButton) !== false) {
              $tag = ArrayHelper::remove($closeButton, 'tag', 'button');
              $label = ArrayHelper::remove($closeButton, 'label', 'Close');
              if ($tag === 'button' && !isset($closeButton['type'])) {
                  $closeButton['type'] = 'button';
              }
  
              return Html::tag($tag, $label, $closeButton);
          } else {
              return null;
          }
      }
docs: Renders the close button.
  @return string the rendering result