public function filterByGeneratedByDotmailer($collection, $column)
    {
        $field = $column->getFilterIndex() ? $column->getFilterIndex()
            : $column->getIndex();
        $value = $column->getFilter()->getValue();
        if ($value == 'null') {
            $collection->addFieldToFilter($field, ['null' => true]);
        } else {
            $collection->addFieldToFilter($field, ['notnull' => true]);
        }
    }