Custom Submit Callback only triggering when Form is Valid when in Dialog

I'm trying to create a node in a dialog with the following link:

Enter the title of a registration form or <a class="use-ajax" 
    data-dialog-options="{&quot;width&quot;:800}" 
    data-dialog-type="modal" 
    href="/node/add/mycontenttype?enableAjaxSubmit=true">
    create a new registration form.
</a>

The callback _mymodule_close_dialog() runs perfectly when there are no validation errors. But when there is an error (e.g. the title is empty), JS just outputs an error Uncaught TypeError: this.url is undefined.

I can get rid of the JS error by preventing the form actions from becoming model buttons, but then the form just silently fails.

How can I show a form's error messages when creating a node in a dialog?

The callback:

/**
 * Implements hook_form_BASE_FORM_ID_alter() for \Drupal\node\NodeForm.
 */
function mymodule_form_node_mycontenttype_form_alter(&$form, FormStateInterface $form_state) {
    $enableAjaxSubmit = \Drupal::request()->query->get('enableAjaxSubmit');
    if (!empty($enableAjaxSubmit) && $enableAjaxSubmit === 'true') {    
      $form['#attached']['library'][] = 'core/drupal.dialog.ajax';
      $form['#attached']['library'][] = 'core/jquery.form';

      $form['actions']['submit']['#attributes']['class'][] = 'use-ajax-submit';
      $form['actions']['submit']['#submit'][] = '_mymodule_close_dialog';
    }
}

use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Ajax\CloseModalDialogCommand;
use Drupal\Core\Ajax\PrependCommand;
use Drupal\Core\Ajax\AlertCommand;

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

  // if (!$form_state->getErrors()) {
    $response->addCommand(new CloseModalDialogCommand());
  // }

  $messages = ['#type' => 'status_messages'];
  $response->addCommand(new PrependCommand('.some-wrapper', $messages));

  $form_state->setResponse($response);
}

I followed:

• mostly: https://agaric.coop/blog/display-forms-modal-dialog-drupal-8
• and a bit of: https://www.drupal.org/project/drupal/issues/2934463#comment-13180158

Update

Changed:

$form['actions']['submit']['#submit'][] = '_mymodule_close_dialog';

To:

$form['actions']['submit']['#ajax']['callback'] = '_mymodule_close_dialog'; // Rename function to remove `_`
$form['actions']['submit']['#ajax']['url'] = \Drupal\Core\Url::fromRoute('node.add', ['node_type' => $contentType]);
$form['actions']['submit']['#ajax']['options'] = [
  'query' => [
    \Drupal\Core\Form\FormBuilderInterface::AJAX_FORM_REQUEST => TRUE,
  ],
];

But getting a PHP error:

Symfony\Component\HttpKernel\Exception\HttpException: The specified #ajax callback is empty or not callable. in Drupal\Core\Form\FormAjaxResponseBuilder->buildResponse()

When I debug($triggering_element) in the FormAjaxResponseBuilder file, the submit element did not have an #ajax key or the use-ajax-submit class. Thus, the triggering element is not ultimately altered.

Any ideas?

and a bit of: https://www.drupal.org/project/drupal/issues/2934463#comment-13180158

You missed adding this part to your alter, which should fix your url undefined problem.

$form['actions']['submit']['#ajax']['url'] = \Drupal\Core\Url::fromRoute('node.add');
$form['actions']['submit']['#ajax']['options'] = [
  'query' => [
    Drupal\Core\Form\FormBuilderInterface::AJAX_FORM_REQUEST => TRUE,
  ],
];