Call to a member function givePermissionTo() on null

PHP Snippet 1:

Auth::user()->givePermissionTo('edit articles');

PHP Snippet 2:

public function __construct()
    {
        $this->middleware('auth');
    }

PHP Snippet 3:

Auth::user()->givePermissionTo('edit articles');

PHP Snippet 4:

$user = Auth::user();
$user->givePermissionTo('edit articles);