Paytabs Laravel Package doesn't send callback

Question

My config/paytabs.php file

    'region' => env('paytabs_region', 'SAU'),

    'callback' => env('paytabs_ipn_callback', App\Http\Controllers\PaymentController::class ),

];

My App\Http\Controllers\PaymentController file

use Paytabscom\Laravel_paytabs\Facades\paypage;

class PaymentController extends Controller
{
    public function payment()
    {
        $pay= paypage::sendPaymentCode('all')

            ->sendTransaction('sale')

            ->sendCart('product_id',1,'Product name')

            ->sendCustomerDetails('Name Surname', '[email protected]', '1111110011', 'test', 'City', 'Cairo', 'EG', '1234','100.279.20.10')

            ->sendHideShipping(true)

            ->sendURLs('https://testsite.com', 'https://testsite.com/paymentIPN')

            ->sendLanguage('en')

            ->create_pay_page();

        return $pay;
    }

    public function updateCartByIPN( $requestData){
        $data = $requestData->getTranRef();
    }
}

I didn't change any route or function in package. I am not receiving any callback and it directly redicerts to sendURL. What is problem or if there is any error how can I see it. Package : https://github.com/paytabscom/paytabs-php-laravel-package

---

I found a problem in Paytabscom\Laravel_paytabs\Controllers\PaytabsLaravelListenerApi.php which is :

if(is_object($callback) && method_exists($callback, 'updateCartByIPN') ){
    $callback->updateCartByIPN($ipnRequest);
}

Problem is about the is_object function. I do not still know why this function returns false when $callback have value of App\Http\Controllers\PaymentController.