Как я могу загрузить и отправить форму через Ajax в Symfony 4

Я загружаю форму через Ajax на мою страницу:

$('.table tbody').on( 'click', '.edit-item', function (e) {
var id = $(this).attr("data-id");
e.preventDefault();
var $link = $(e.currentTarget);
$.ajax({
method:'POST',
data: {
"id": id
},
url: $link.attr('href')
}).done(function(data){
$('.form-output').html(data.output);

$('#form_save').on( 'click', function (e) {
e.preventDefault();
var form = $(this).closest('form');
var formData = form.serialize();
alert("save button clicked");
$.ajax({
method:'POST',
url:'{{ path('edit_form', { 'slug': page.slug }) }}',
data: formData,
success: function(data){

alert("success");
}
});
});

});
});

controller.php:

 /**
* @Route("/pages/{slug}/edit", name="edit_form", methods={"POST", "GET"})
*/


public function toggleArticleHeart($slug, Request $request){

if($request->request->get('id')){

$id = $request->request->get('id');
$item = new User();

$item= $this->getDoctrine()->getRepository(User::class)->find($id);
$form = $this->createFormBuilder($item)
->add('username', TextType::class, array('attr' => array('class' => 'form-control')))
->add('email', EmailType::class, array('attr' => array('class' => 'form-control')))
->add('is_active', HiddenType::class)
->add('plainPassword', RepeatedType::class, array('type' => PasswordType::class,'invalid_message' => 'The password fields must match.','options' => array('attr' => array('class' => 'password-field')),'required' => false,'first_options'  => array('label' => 'Passwort', 'attr' => array('class' => 'form-control')),'second_options' => array('label' => 'Passwort wiederholen', 'attr' => array('class' => 'form-control')),))
->add('cancel', ButtonType::class, array('label' => 'Abbrechen','attr' => array('class' => 'cancel form-btn btn btn-default pull-right close_sidebar close_h')))
->add('save', SubmitType::class, array('label' => 'Speichern','attr' => array('id' => 'submit-my-beautiful-form','class' => 'form-btn btn btn-info pull-right','style' => 'margin-right:5px')))
->getForm();
$form->handleRequest($request);


$response = new JsonResponse(
array(
'message' => 'Success',
'output' => $this->renderView('form.html.twig',
array(
'entity' => $item,
'form' => $form->createView(),
))), 200);

return $response;

} else {

$data = $request->request->get('data');
$entityManager->persist($data);
$entityManager = $data->getDoctrine()->getManager();
$entityManager->flush();
$response = new Response();
$response->send();
}
}

Я получаю сообщение «кнопка сохранения нажата», но не сообщение «успех».

И есть также ошибка Ajax 500:

Uncaught PHP Exception ErrorException: «Примечание: неопределенная переменная:
EntityManager»