Set custom element values to strings on error (fix #1691)

These two elements are custom built in common/property-form-batch-edit.phtml
and will POST arrays. The form knows about the elements via hidden inputs,
which expect strings. If the form has an error, we must set the input values
to empty strings or there will be errors.

application/src/Controller/Admin/ItemController.php

@@ -295,6 +295,8 @@ public function batchEditAction()
                 $this->messenger()->addSuccess('Items successfully edited'); // @translate
                 return $this->redirect()->toRoute(null, ['action' => 'browse'], true);
             } else {
+                $form->get('set_value_visibility')->setValue('');
+                $form->get('value')->setValue('');
                 $this->messenger()->addFormErrors($form);
             }
         }