Symfony 4 - The key "_username" must be a string, "NULL" given

my project symfony 4 I have a function that serves me for authentication but also for registration:

/**
     * Login
     *
     * @Route("/login", name="account_login")
     * 
     * @return Response
     */
    public function login2(AuthenticationUtils $utils, Request $request, ObjectManager $manager, UserPasswordEncoderInterface $encoder)
    {
        // Connexion
        $error = $utils->getLastAuthenticationError();
        $username = $utils->getLastUsername();

        // Inscription
        $user = new User();
        $form = $this->createForm(RegistrationType::class, $user);
        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {
            $password = $encoder->encodePassword($user, $user->getPassword());
            $user->setHash($password);
            $manager->persist($user);
            $manager->flush();

            $this->addFlash(
                'success',
                'Votre compte a bien été créé ! Vous pouvez maintenant vous connecter'
            );

            $this->redirectToRoute('account_login');
        }


        return $this->render('security/login.html.twig', [
            'hasError' => $error !== null,
            'username' => $username,
            'form' => $form->createView(),
        ]);
    }

So in my twig file I have my login form:

<form id="login-form" action="{{path('account_login')}}" method="post" role="form" style="display: block;">
     <div class="form-group">
          <label for="username">Email</label>
          <input type="text" name="_username" id="username" tabindex="1" class="form-control" placeholder="Adresse email" required value={{username}}>
     </div>
     <div class="form-group">
         <label for="password">Mot de passe</label>
         <input type="password" name="_password" id="password" tabindex="2" class="form-control" placeholder="Mot de passe">
     </div>
</form>

And my registration form :

{{form_start(form)}}
{{form_widget(form)}}
{{form_end(form)}}

But when I submit my registration form, I've this error :

The key "_username" must be a string, "NULL" given.

In my security.yml I've :

form_login:
        login_path: account_login
        check_path: account_login
        default_target_path: absence_index

      logout:
        path: account_logout
        target: account_login

I found this post : The key "_username" must be a string, "NULL" given symfony 4

It's the same problem I think, but I don't understand what's the solution.