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
}