Dropdown Ajax onchange SonataAdminBundle Symfony2 Issue

I am trying to implement onchange dropdown in SonataAdminBundle. My Entity is like

 class BuilderHomePage
{
/**
 * @var integer
 *
 * @ORM\Column(name="id", type="integer", nullable=false)
 * @ORM\Id
 * @ORM\GeneratedValue(strategy="IDENTITY")
 */
private $id;

 /**
 * @var \Hello\FrontendBundle\Entity\MtContentType
 *
 * @ORM\ManyToOne(targetEntity="Hello\FrontendBundle\Entity\MtContentType")
 * @ORM\JoinColumns({
 *   @ORM\JoinColumn(name="content_type_id", referencedColumnName="id")
 * })
 */
private $section;


/**
 * @var string
 *
 * @ORM\Column(name="title", type="string",length=100, nullable=false)
 */
private $title;

My Admin Class

     public function getTemplate($name)
     {
       switch ($name) {
        case 'edit':
            if ($this->getSubject()->getId()) {
                return 'HelloAdminBundle:Builder:base_edit.html.twig';
            } else {
                return 'HelloAdminBundle:Builder:base_edit.html.twig';
            }
            break;
        default:
            return parent::getTemplate($name);
            break;
    }

}

protected function configureRoutes(RouteCollection $collection) {
$collection
    ->add('getArticleFromSection', 'getArticleFromSection')
    ;
}
protected function configureFormFields(FormMapper $formMapper)
{
    $formMapper

        ->add('section')
        ->add('title','choice',array('required' => false ))

    ;
}

My Builder:base_edit.html.twig

     <script type="text/javascript">
     $(document).ready(function()
       {
         $("#{{ admin.uniqId }}_section").change(function()
        {
        var id=$(this).val();
        var dataString = 'id='+ id;
        $.ajax
        ({
            type: "POST",
            url: "{{ admin.generateObjectUrl('getArticleFromSection', object) }}",
            data: dataString,
            cache: false,
            success: function(html)
            {
                $("#{{ admin.uniqId }}_title").html(html);
            } 
        });
       });

     });

</script>

Ajax Request Controller

    $article        = $this->get('hello_frontend.article');
    $totalArticle        = $article->getArticleByContentType($id);

     $html = "";
    foreach($totalArticle as $res){
    $html .="<option value=".$res->getId().">".$res->getTitle()."</option>";
    }

Till now everything works fine....

But when i tried to click on create.its showing an error

I am not able to figure out the problem. your help will be appreciated