/**
  * @return string
  */
 protected function renderHiddenRecipientsInputForValidationContent()
 {
     $hiddenInputId = Element::resolveInputIdPrefixIntoString(array_merge($this->inputPrefixData, array('recipientsValidation')));
     $hiddenInputName = Element::resolveInputNamePrefixIntoString(array_merge($this->inputPrefixData, array('recipientsValidation')));
     $idInputHtmlOptions = array('id' => $hiddenInputId);
     $id = Element::resolveInputIdPrefixIntoString(array_merge($this->inputPrefixData, array('recipientsValidation')));
     $content = ZurmoHtml::hiddenField($hiddenInputName, null, $idInputHtmlOptions);
     $content .= $this->form->error($this->model, 'recipientsValidation', array('inputID' => $hiddenInputId), true, true, $id);
     return $content;
 }