How submit form in modal ?

[moreaul]

I integrate a form in the modal in order to create a new object but the validation of this one does not happen as agreed:
the object is not created and for a few seconds the redirect page is loaded in the modal.

Button on index.html.twig :

<a href="{{ path('category_new') }}" data-sideclick-modal-trigger>
            <i class="fa fa-plus-circle"></i> Créer une catégorie
        </a>

Here is my controller code:

/**
      * @Route("/new-category", name="category_new", methods={"GET", "POST"})
      */
     public function addCategory(Request $request): Response
     {
         $category = newCategory();

         $form = $this->createForm(CategoryType::class, $category);
         $form->handleRequest($request);

         if ($form->isSubmitted() && $form->isValid()) {
             $this->entityManager->persist($category);
             $this->entityManager->flush();

             return $this->redirectToRoute('stores_index');
         }

         return $this->renderForm('store/category_form.html.twig', [
             'category' => $category,
             'form' => $form,
         ]);
}

Here my category_form.html.twig :

<div class="modal-content">
            <div class="modal-header">
                <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
                <h4 class="modal-title">Modal title</h4>
            </div>
            <div class="modal-body">
                {{ form_start(form) }}
                {{ form_widget(form) }}
                <button class="btn">{{ button_label|default('Save') }}</button>
                {{ form_end(form) }}
            </div>
            <div class="modal-footer">
                <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
                <button type="button" class="btn btn-primary">Save changes</button>
            </div>
        </div><!-- /.modal-content -->