fix $success_response in FolderController

Author: streamtw

src/controllers/FolderController.php

@@ -61,7 +61,7 @@ public function getAddfolder()
             return parent::error('folder-alnum');
         } else {
             parent::createFolderByPath($path);
-            return $this->success_response;
+            return parent::$success_response;
         }
     }
 }