Perform any necessary changes to the form values prior to storage. There is no need for this function to actually store the data.
Overrides HandlerBase::submitGroupByForm
public function submitGroupByForm(&$form, &$form_state) {
parent::submitGroupByForm($form, $form_state);
$item =& $form_state['handler']->options;
// Add settings for "field API" fields.
$item['group_column'] = $form_state['values']['options']['group_column'];
$item['group_columns'] = array_filter($form_state['values']['options']['group_columns']);
}