Symfony 3 Form component - my entity constructor has arguments

Question

I have an entity called User and the form to register a new user. The problem I am having is that my entity has arguments in its constructor, so I can not instantiate a new empty User that has no data in it and pass it to the form builder in the controller. When I use default options on my form I then get the error message

Warning: Missing argument 1 for User\User::__construct(), called in /mnt/project/vendor/symfony/symfony/src/Symfony/Component/Form/Extension/Core/Type/FormType.php on line 136 and defined

I do not want to allow User object be constructed without mandatory things like email or name. What's the solution here?

The form:

class UserRegisterForm extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('email', TextType::class)
            ->add('password', PasswordType::class)
            ->add('firstName', TextType::class)
            ->add('lastName', TextType::class)
            ->add('save', SubmitType::class);
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => User::class
        ));
    }
}

My entity:

/**
 * @ORM\Entity
 */
class User implements UserInterface
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id;

    /**
     * @ORM\Column(type="string")
     * @Assert\Email(
     *     message = "The email '{{ value }}' is not a valid email.",
     *     checkMX = true
     * )
     */
    protected $email;

    /**
     * @ORM\Column(type="string")
     * @Assert\NotBlank()
     * @Assert\Length(min=3)
     */
    protected $password;

    /**
     * @ORM\Column(type="string")
     * @Assert\NotBlank()
     */
    protected $firstName;

    /**
     * @ORM\Column(type="string")
     * @Assert\NotBlank()
     */
    protected $lastName;

    public function __construct($email, $firstName, $lastName)
    {
        $this->email = $email;
        $this->firstName = $firstName;
        $this->lastName = $lastName;
    }
}

And the controller

public function registerView(Request $request)
{
    $form = $this->formFactory
        ->createBuilder(UserRegisterForm::class)
        ->getForm();

    $form->handleRequest($request);

    if ($form->isSubmitted() && $form->isValid()) {
        die("Valid form, the user entity is going to be saved here");
    }

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

Update Found this - Symfony2 Forms - How to use parametrized constructors in form builders. it does not solve the problem though because I do not want to pass any dummy data to the constructor. Not passing the object to the form builder causes it to automatically bypass the whole validation.