session keeps randomly giving Undefined array key while key is defiend

It was running fine on my local host and when i first hosted the site online it was working fine but when i changed the host i keep randomly getting PHP Warning: Undefined array key "UID" when the key UID is defined because the pages wont open with out it

if (session_status() == PHP_SESSION_NONE) {
            session_start();
        }
          
          $sqlrule = "SELECT * FROM `user` WHERE  userid = ".$_SESSION['UID'];
                   

That is how i user it i know i can check if it is set but i wont to know what causes the issue