code: public function renderHeader()
      {
          if ($this->header !== false || !empty($this->header)) {
              $content = ArrayHelper::getValue($this->header, 'label', '');
              $options = ArrayHelper::getValue($this->header, 'options', ['class' => 'header']);
              $header = Ui::tag('div', $content, $options);
              $lines[] = Ui::beginTag('div', ['class' => 'content']);
              $lines[] = $header;
              return implode("\n", $lines);
          } else {
              return null;
          }
      }
docs: Renders the header (if any)
  
  @return null|string