Drupal 8 custom registration form

I try to build a custom registration form which should be displayed in a custom block and I don't want to insert the normal registration form and alter it via a hook or use an extension like form_block, because I want to learn the ways how drupal 8 works with forms.

My block looks like this:

<?php

namespace Drupal\ajax_registration_form\Plugin\Block;

use Drupal\ajax_registration_form\Form\AjaxRegistrationForm;
use Drupal\Core\Block\BlockBase;

/**
 * Provides a 'AjaxRegistrationBlock' block.
 *
 * @Block(
 *  id = "ajax_registration_block",
 *  admin_label = @Translation("Ajax registration block"),
 * )
 */
class AjaxRegistrationBlock extends BlockBase {

  /**
   * {@inheritdoc}
   */
  public function build() {

    $content = \Drupal::formBuilder()->getForm(AjaxRegistrationForm::class);

    return $content;
  }

}

My custom registration form looks like this:

<?php

namespace Drupal\ajax_registration_form\Form;

use Drupal\Core\Form\FormStateInterface;
use Drupal\user\RegisterForm;



class AjaxRegistrationForm extends RegisterForm {


  /**
   * {@inheritdoc}
   */
  public function form(array $form, FormStateInterface $form_state) {

    return parent::form($form, $form_state);
  }
}

I just tried to extend the normal RegisterForm and in the first step I just wanted to return the parent form to see if it works. But it doesn't...

Error message:

Fatal error: Call to a member function getEntityTypeId() on null in /Users/*******/Sites/priv/radweiser/web/core/lib/Drupal/Core/Entity/EntityForm.php on line 77

I think it's the missing user entity in the form, but I don't know how I can "put" this entity in my form.

I found the solution in the code of the formblock module.

I altered my block to something like this:

<?php

namespace Drupal\ajax_registration_form\Plugin\Block;

use Drupal\Core\Annotation\Translation;
use Drupal\Core\Block\Annotation\Block;
use Drupal\Core\Block\BlockBase;
use Drupal\Core\Entity\EntityFormBuilderInterface;
use Drupal\Core\Entity\EntityManagerInterface;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\Core\Session\AccountInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Provides a 'AjaxRegistrationBlock' block.
 *
 * @Block(
 *  id = "ajax_registration_block",
 *  admin_label = @Translation("Ajax registration block"),
 * )
 */
class AjaxRegistrationBlock extends BlockBase implements ContainerFactoryPluginInterface {


  /**
   * The entity manager
   *
   * @var \Drupal\Core\Entity\EntityManagerInterface.
   */
  protected $entityManager;

  /**
   * The entity form builder
   *
   * @var \Drupal\Core\Entity\EntityManagerInterface.
   */
  protected $entityFormBuilder;

  /**
   * Constructs a new UserRegisterBlock plugin
   *
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin_id for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Entity\EntityManagerInterface $entityManager
   *   The entity manager.
   * @param \Drupal\Core\Entity\EntityFormBuilderInterface $entityFormBuilder
   *   The entity form builder.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityManagerInterface $entityManager, EntityFormBuilderInterface $entityFormBuilder) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->entityManager = $entityManager;
    $this->entityFormBuilder = $entityFormBuilder;
  }

  /**
   * @inheritdoc
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container->get('entity.manager'),
      $container->get('entity.form_builder')
    );
  }


  /**
   * Implements \Drupal\block\BlockBase::build().
   */
  public function build() {
    $build = array();

    $account = $this->entityManager->getStorage('user') ->create(array());
    $build['form'] = $this->entityFormBuilder->getForm($account, 'register');

    $build['form']['account']['mail']['#description'] = t('');
    kint($build['form']['account']);

    return $build;
  }

  /**
   *Implements \Drupal\block\BlockBase::blockAccess().
   *
   * @param \Drupal\Core\Session\AccountInterface $account
   *
   * @return bool|\Drupal\Core\Access\AccessResult
   */
  public function blockAccess(AccountInterface $account) {
    return ($account->isAnonymous()) && (\Drupal::config('user.settings')->get('register') != USER_REGISTER_ADMINISTRATORS_ONLY);
  }

}

Now I can alter the form and implement ajax logic using the Form Api (alter the mail input description as example)