code: protected function renderFooter()
      {
          if ($this->footer !== null) {
              $content = $this->footer;
              if($this->usebutton){
                  $content .= $this->renderButton(self::POSITION_FOOTER);
              }
              return Html::tag('div', "\n" . $content . "\n", ['class' => 'box-footer clearfix']);            
          } else {
              return null;
          }
      }
docs: Renders the HTML markup for the footer of the modal
  @return string the rendering result