kartik\export\ExportMenu::findGroupedColumn PHP Method

    protected function findGroupedColumn()
    {
        foreach ($this->getVisibleColumns() as $key => $column) {
            if (isset($column->group) && $column->group == true) {
                $this->_groupedColumn[$key] = ['firstLine' => -1, 'value' => null];
            } else {
                $this->_groupedColumn[$key] = null;
            }
        }
        $this->_groupedColumn[] = null;
        //prevent the overflow
        $this->_groupedColumn[] = null;
        //prevent the overflow
    }