Call to a member function tasks() on null on laravel 5

Let me explain with a couple of word my problem. On my controller i have this line:

$tasks = $student->group->tasks()->orderBy('created_at', 'desc')->withPivot('id')->get();

This works for existing users, but when i try to create new ones i receive

Call to a member function tasks() on null

Can i with something like this or what do you suggest ?

if(!is_null($tasks))
    $tasks = $student->group->tasks()->orderBy('created_at', 'desc')->withPivot('id')->get();
}

via Chebli Mohamed