cakePHP 1.3：使用Ajax渲染问题

I want a elements conent as string in my ajax response.

so i have put the below code in my controller action

$view = new View($this, false);
$view->viewPath = 'elements/private_challenge/';            
$res = $view->render('supporters_list_supporter');

now in my supporters_list_supporter.ctp i have put one form as below:

echo $this->Form->create('RequestFrHelpReply',array('action'=>'add_helpreq_reply','class'=>'form','id' => 'helprequestreplyaddfrm','name'=>'helprequestreplyaddfrm')); 

 echo $this->Form->hidden('PrivateChallengeRequestHelp.ReqID',array('value'=>$supporterlist['RequestFrHelp']['REQID']));
 echo "<div class='alignleft'>";
 echo $this->Form->input('PrivateChallengeRequestHelp.txt_helpreq_reply', array('label'=>false,'type' => 'textarea', 'id'=>'txt_helpreq_reply','escape' => false, 'div' => false, "class"=>"textarea request-help-textarea",'onKeyDown'=>'textCounterComment(this.form.txt_helpreq_reply,this.form.remLenth,'.Configure::read('CommentSetting.comment_body_max_limit').',\'remLenthDiv\');', 'onKeyUp'=>'textCounterComment(this.form.txt_helpreq_reply,this.form.remLenth,'.Configure::read('CommentSetting.comment_body_max_limit').',\'remLenthDiv\');'));
  echo "</div>";
  echo '<span class="request-arrow"></span><span class="user-name"> : '.$html->get_users_username($session->read('Auth.User.id')).'</span>';
 echo $html->link(__l('Reply'),'#',array('id'=>'helpreq_reply_link','class'=>'request-help-button alignleft','title'=>__l('Reply'),'escape'=>false,'onclick'=>'helpreq_reply(\'reply\');return false;'));               
 echo "</div>";     
 echo $this->Form->end();

So its giving me error related to form helper missing so can anybody help me how to add form helper with this exetrnal object?

Updated: Error Message :

   $view = stdClass
   stdClass::$modelScope = false
   stdClass::$entityPath = "RequestFrHelpReply."</pre><pre class="stack-trace">Helper::setEntity() - CORE\cake\libs\view\helper.php, line 422
   FormHelper::create() - CORE\cake\libs\view\helpers\form.php, line 217
   include - APP\views\elements\private_challenge\help_request_form.ctp, line 21

Have you got

public $helpers = array('Form');

in your controller?