Modify FronController with use class_name in is_subclass_of.

app/FrontController.php

@@ -143,7 +143,7 @@ class FrontController
             $layout = new $layout_class();
             $layout->setException($e);
             $error_action = new ErrorAction($e);
-            if (isset($action) && is_subclass_of($action, 'AjaxAction')) {
+            if (isset($action_class) && is_subclass_of($action_class, 'AjaxAction')) {
                 $error_action->setAjaxError();
             }
             return $layout->fetch($error_action);