Undefined index: group_by in quiz_views_handler_filter_quiz_nid->construct() line 46

Notice: Undefined index: group by in quiz_views_handler_filter_quiz_nid->construct() (line 46 of ...\quiz\includes\views\handlers\quiz_views_handler_filter_quiz_nid.inc).

The group_by property is not initalized.

I try to change these codes to solve this problem:

function construct() {
    parent::construct();
    /*
     *  $this->vid_field = !empty($this->definition['vid field']) ? $this->definition['vid field'] : 'vid';
    $this->group_by = $this->definition['group by'];
    if (!empty($this->definition['secondary group by'])) {
      $this->secondary_group_by = $this->definition['secondary group by'];
    }
     */
  }

--> AND Add function :

 function init(&$view, &$options) {
      parent::init($view, $options);
      $this->vid_field = !empty($this->definition['vid field']) ? $this->definition['vid field'] : 'vid';
    $this->group_by = $this->definition['group by'];
    if (!empty($this->definition['secondary group by'])) {
      $this->secondary_group_by = $this->definition['secondary group by'];
    }
  }

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System :
Drupal 7.16
My SQL: 5.1
PHP 5.2.8
Apache 2.2
Windows 7 Pro 64 bits.