Problem with cyrilic encription in URL

It`blow up this message:

A PHP Error was encountered
Severity: Warning

Message: Trying to access array offset on value of type null

Filename: controllers/Posts.php

Line Number: 25

Backtrace:

File: C:\xampp\htdocs\ciblog\application\controllers\Posts.php
Line: 25
Function: _error_handler

File: C:\xampp\htdocs\ciblog\index.php
Line: 315
Function: require_once

404 Page Not Found
The page you requested was not found.