public function renderTableFooter()
    {
        $cells = [];
        foreach ($this->columns as $column) {
            /* @var $column TreeColumn */
            $cells[] = $column->renderFooterCell();
        }
        $content = Html::tag('tr', implode('', $cells), $this->footerRowOptions);
        return "<tfoot>\n" . $content . "\n</tfoot>";
    }