Signup Error

I have set up in aMember admin CP that when a user logs in, it get redirected to one of page X in our website. I have also set up my website, so that when a new user signs up, it automatically logs into system after sign-up completion. However, before activating the account, this error appears on top of page X:
Undefined index: user_id in /var/www/html/dev/amember/library/Am/Lite.php on line 613. (in getUserField($name) in the first if statement.)
But after activation, no such error is shown.

Can anybody tell me what the problem is and how I can solve it?
Thanks.

I changed the code to

Code:

protected function getUserField($name)
    {
        if ($this->hasIdentity())
        {
            $user = Am_Lite::getInstance()->getUser();
            $user_id = $user['user_id'];
            return $user[$name];
        }
        else
        {
            return null;
        }
    }

Is this what you suggested? Because this did not remove the error.