Drupal 8 form submit show "Success! Dialogbox".

Drupal 8 form submit show "Success! Dialogbox".

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Ajax\OpenModalDialogCommand;
use Drupal\Core\Ajax\CloseModalDialogCommand;
use Drupal\Core\Ajex\RedirectCommand;
use Drupal\Core\Ajex\HtmlCommand;
use Drupal\Core\Url;
use Drupal\Core\Ajex\ReplaceCommand;


function modual_form_example_form_alter( array &$form, FormStateInterface $form_state, $form_id ) {
  if ($form_id == 'user_register_form' ) {
    $form['actions']['submit']['#ajax'] = ['callback' => '_submit_message'];

    $form['#attached']['library'][] = 'core/drupal.dialog.ajax';
    $form['#attached']['library'][] = 'core/jquery.form';

    return $form;
  }
}

function _submit_message(array $form, FormStateInterface $form_state) {
    $response = new AjaxResponse();

    // If there are any form errors, re-display the form.
    if ($form_state->hasAnyErrors()) {
      $response->addCommand(new ReplaceCommand('#user_register_form', $form));
    }
    else {
      $response->addCommand(new OpenModalDialogCommand("Success!", 'The modal form has been submitted.', ['width' => 800]));
    }

    return $response;
  }