Fix Child "_delete" does not exist.

If current role is not allowed to delete the item the `_delete` checkboxes are not added in items.

src/Form/Listener/CollectionOrderListener.php

@@ -67,7 +67,7 @@ public function onSubmit(FormEvent $event)
 
         /** @var $item FormBuilder */
         foreach ($form->get($this->name) as $key => $item) {
-            if ($item->get('_delete')->getData()) {
+            if ($item->has('_delete') && $item->get('_delete')->getData()) {
                 // do not re-add a deleted child
                 continue;
             }