Issue #3478276 by roderik, useernamee: Can not add fields to custom elements display

modules/custom_elements_ui/src/Form/EntityCustomElementsDisplayEditForm.php

@@ -4,6 +4,7 @@ namespace Drupal\custom_elements_ui\Form;
 
 use Drupal\Component\Plugin\Factory\DefaultFactory;
 use Drupal\Component\Plugin\PluginManagerBase;
+use Drupal\Core\Ajax\AjaxResponse;
 use Drupal\Core\Ajax\ReplaceCommand;
 use Drupal\Core\Entity\EntityDisplayRepositoryInterface;
 use Drupal\Core\Entity\EntityFieldManagerInterface;
@@ -805,12 +806,17 @@ class EntityCustomElementsDisplayEditForm extends EntityDisplayFormBase {
    * {@inheritdoc}
    */
   public function multistepAjax($form, FormStateInterface $form_state) {
-    $response = parent::multistepAjax($form, $form_state);
-
     $trigger = $form_state->getTriggeringElement();
     if ($trigger['#name'] == 'add_select') {
+      // Replace the whole table. (Don't call parent, which can only handle
+      // its own known '#op's.)
+      $response = new AjaxResponse();
+      $response->addCommand(new ReplaceCommand('#field-display-overview-wrapper', $form['fields']));
       $response->addCommand(new ReplaceCommand('.js-form-item-add-select', $form['add_select']));
     }
+    else {
+      $response = parent::multistepAjax($form, $form_state);
+    }
 
     return $response;
   }