Symfony Object of class Closure could

bonjour mes amis,

j'ai ce probléme lorsque j'affiche le formulaire du création du groupe, j'en sais pas comment le résoudre. j'utilise la version 2.7

** l'erreur:
Catchable Fatal Error: Object of class Closure could not be converted to string
500 Internal Server Error - ContextErrorException

Stack Trace
in vendor/symfony/symfony/src/Symfony/Component/Form/ChoiceList/Factory/DefaultChoiceListFactory.php at line 166 -
// If "choice_label" is set to false and "expanded" is true, the value false
// should be passed on to the "label" option of the checkboxes/radio buttons
$dynamicLabel = call_user_func($label, $choice, $key, $value);
$label = false === $dynamicLabel ? false : (string) $dynamicLabel;
}
$view = new ChoiceView(

-Voilà mon GroupsType:
class GroupsType extends AbstractType
{
/**
* @param FormBuilderInterface $builder
* @param array $options
*/
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->add('image', new ImagesType())
->add('name_groups','text',array('required' => true, 'attr' => array('placeholder' => 'Nom du groupe')))
->add('role', new RolesType())
;
}
-Voilà mon RoleType:
lass RolesType extends AbstractType
{
/**
* @param FormBuilderInterface $builder
* @param array $options
*/

public function buildForm(FormBuilderInterface $builder, array $options)
{

    $builder
       ->add('groups', 'choice', array(
                                         'choices' => array(
                                                              'class' => 'GroupsBundle:Roles',
                                                              'query_builder' => function(EntityRepository $er)
                                                                                {
                                                                                    return $er->createQueryBuilder('r')
                                                                                    ->orderBy('r.id', 'ASC');
                                                                                 },
                                                              'required' => false,
                                                               'multiple' => false,
                                                               'expanded' => false,
                                                               'placeholder' => 'Choisir le role du votre groupe'

                                                            )
                                      )
            )

       ->add('permissions_role','entity',array(
               'class' => 'GroupsBundle:Permissions',
               'multiple' => true,
               'expanded' => true,
               'query_builder' => function(EntityRepository $er) {
                   return $er->createQueryBuilder('u')
                       ->orderBy('u.name_permissions', 'ASC');
               },
               'required' => true
           )
           )

    ;
}

Merci d'avance ;)