Symfony 5, form error Twig\Environment::getTemplateClass() - forms

Question

I'm trying to create a registration form and add the form to the template for documentation
Controller:
class RegController extends AbstractController
{
#[Route('/register', name: 'app_register')]
public function register(Request $request, UserPasswordEncoderInterface $passwordEncoder): Response
{
$user = new Admin();
$form = $this->createForm(RegistrationFormType::class, $user);
return $this->render('registration/reg.html.twig', [
'form' => $form->createView()
]);
}
}
Form type:
class RegistrationFormType extends AbstractType
{
public function buildForm(FormBuilderInterface $builder, array $options): void
{
$builder
->add('email', EmailType::class)
->add('plainPassword', PasswordType::class, [
'mapped' => false,
'attr' => ['autocomplete' => 'new-password'],
'constraints' => [
new NotBlank([
'message' => 'Please enter a password',
]),
new Length([
'min' => 6,
'minMessage' => 'Your password should be at least {{ limit }} characters',
'max' => 4096,
]),
],
])
;
}
public function configureOptions(OptionsResolver $resolver): void
{
$resolver->setDefaults([
'data_class' => Admin::class,
]);
}
}
Template:
{% extends 'base.html.twig' %}
{% block title %}{{ 'Register' | trans }}!{% endblock %}
{% block body %}
<main class="form-signin admin-register" style="max-width: 300px">
{{ form(form) }}
</main>
{% endblock %}
Error:
Twig\Environment::getTemplateClass(): Argument #1 ($name) must be of type string, null given
Question:
It seems to have done everything according to the documentation, why can the following error occur?