protected function renderHeader()
    {
        $button = $this->renderCloseButton();
        if ($button !== null) {
            $this->header = $this->header . "\n" . $button;
        }
        if ($this->header !== null) {
            Html::addCssClass($this->headerOptions, ['widget' => 'modal-header']);
            return $this->htmlHlp->tag('div', "\n" . $this->header . "\n", $this->headerOptions);
        } else {
            return null;
        }
    }