Ajax error on node form edit page I'm trying to ajaxify part of node form. I have a hook_form_alter that adds extra ajax property to a widget of select field. The problem I'm facing is the callback method / function, which is not fired when change is happening. 

So this is my form_alter code which resides in accountancy_invoice/accountancy_invoice.module:

function accountancy_invoice_form_node_form_alter(array &amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  if ($form_id == 'node_invoice_form') {

    $form['accommodation'] = array(
      '#type' =&gt; 'container',
      '#prefix' =&gt; '&lt;div id="user-full-name"&gt;Dave&lt;/div&gt;',
      '#attributes' =&gt; array(
        'class' =&gt; 'user-full-name',
      ),
    );

    $form['field_invoice_customer']['widget']['#ajax'] = array(
      'callback' =&gt; 'Drupal\accountancy_invoice\Form\AjaxifyForm::customerValues',
      'progress' =&gt; array(
        'type' =&gt; 'throbber',
        'message' =&gt; NULL,
      )
    );

  }
}


This is my AjaxifyForm class which resides in accountancy_invoice/src/Form/AjaxifyForm.php:

namespace Drupal\accountancy_invoice\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Ajax\HtmlCommand;
use Drupal\Core\Ajax\ReplaceCommand;
use Drupal\Core\Ajax\OpenModalDialogCommand;

class AjaxifyForm {
  public static function customerValues(array &amp;$form, \Drupal\Core\Form\FormStateInterface $form_state) {
    $response = new AjaxResponse();
    $response-&gt;addCommand(new ReplaceCommand('#user-full-name', '&lt;span class="unit"&gt;Bryan&lt;/span&gt;'));
    $title = "Example Dialog";
    $content = "foo bar content";
    $options = array();
    $response-&gt;addCommand(new OpenModalDialogCommand($title, $content, $options));
    return $response;
  }
}


It doesn't really matter at the moment what this ajax should do. For testing purposes I just want to change name from Dave to Bryan in #user-full-name and display sample dialog box.

In the console, each time ajax is triggered I got an error: 



This is complete error message (it's trimmed in the screenshot):
↵An AJAX HTTP error occurred.↵HTTP Result Code: 200↵Debugging information follows.↵Path: /node/add/invoice?ajax_form=1↵StatusText: OK↵ResponseText: TypeError: Argument 1 passed to Drupal\Core\Form\FormState::setError() must be of the type array, null given, called in /var/www/drupalvm/drupal/web/core/lib/Drupal/Core/Field/WidgetBase.php on line 441 in Drupal\Core\Form\FormState-&gt;setError() (line 1093 of /var/www/drupalvm/drupal/web/core/lib/Drupal/Core/Form/FormState.php).
This is a generic issue that will show for various reasons, often because of a bug in the code handling the server response. Look in your server logs for more information as to what has happened.