Issue #3394728: Undefined array key "#prefix" and deprecated function:...

Issue #3394728: Undefined array key "#prefix" and deprecated function: explode() in Drupal\file\Element\ManagedFile::uploadAjaxCallback()

Closes #3394728

core/modules/file/src/Element/ManagedFile.php

@@ -181,7 +181,12 @@ public static function uploadAjaxCallback(&$form, FormStateInterface &$form_stat
     /** @var \Drupal\Core\Render\RendererInterface $renderer */
     $renderer = \Drupal::service('renderer');
 
-    $form_parents = explode('/', $request->query->get('element_parents'));
+    $form_parents = explode('/', $request->query->get('element_parents') ?? '');
+
+    // Don't continue if request doesn't have element_parents.
+    if ($form_parents == ['']) {
+      return new AjaxResponse();
+    }
 
     // Sanitize form parents before using them.
     $form_parents = array_filter($form_parents, [Element::class, 'child']);