Argument 1 passed to Symfony\Component\Form\FormRenderer::renderBlock() must be an instance of ...\FormView, instance of ...\Form given

/**
 * @Route("/register", name="registrationPage")
 */
public function showRegistrationPage(Request $request)
{
    $form = $this->createForm(RegisterType::class, (new UserInformation()));

    return $this->render('auth/register.html.twig', [
        'register_form' => $form->createView()
    ]);
}

http://symfony.com/doc/current/forms.html#building-the-form

the missing part was createView() method

/**
 * @Route("/register", name="registrationPage")
 */
public function showRegistrationPage(Request $request)
{
    return $this->render('auth/register.html.twig', [
        'register_form' => $this->createForm(RegisterType::class, (new UserInformation()))->createView()
    ]);
}