private function assertFieldIsStaticSelectbox(Tracker_FormElement $field)
 {
     if (!($field instanceof Tracker_FormElement_Field_Selectbox && $field->getBind() instanceof Tracker_FormElement_Field_List_Bind_Static)) {
         $exception_message = $GLOBALS['Language']->getText('plugin_tracker_formelement_exception', 'field_must_be_static');
         throw new Exception($exception_message);
     }
 }