chore: fix phpstan errors

src/Service/BookingEditSubmitter.php

@@ -13,6 +13,7 @@ use App\Form\Model\BookingDto;
 use Psr\Log\LoggerInterface;
 use Symfony\Component\HttpFoundation\RedirectResponse;
 use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\Session\FlashBagAwareSessionInterface;
 use Symfony\Component\Routing\Generator\UrlGeneratorInterface;
 
 /**
@@ -119,7 +120,10 @@ class BookingEditSubmitter
 
     private function addFlash(Request $request, string $type, string $message): void
     {
-        $request->getSession()->getFlashBag()->add($type, $message);
+        $session = $request->getSession();
+        if ($session instanceof FlashBagAwareSessionInterface) {
+            $session->getFlashBag()->add($type, $message);
+        }
     }
 
     private function redirectToEdit(int $bookingId): RedirectResponse