Codigniter Undefined property: stdClass::$tn

Hi everybody
I have an error in codigniter version 3 project in back and front (this is done when choosing arabic link to translate the site in arabic langage).
Please anyone have some ideas?
A PHP Error was encountered
Severity: Notice

Message: Undefined property: stdClass::$tn

Filename: helpers/MY_string_helper.php

Line Number: 243

Backtrace:

File: C:\wamp64\www\onagri_site\smart\helpers\MY_string_helper.php
Line: 243
Function: _error_handler

File: C:\wamp64\www\onagri_site\smart\views\admin_site\pages.php
Line: 38
Function: Clang