Session Error

Question

How should I solve this kind of problem, since it is new to  me this code igniter. this error

 

A PHP Error was encountered

Severity: Notice

Message: Undefined property: VerifyLogin::$session

Filename: controllers/VerifyLogin.php

Line Number: 30

Fatal error: Call to a member function set_userdata() on null in C:\xampp\htdocs\Student\application\controllers\VerifyLogin.php on line 30