fix: avoid null-pointer exception

src/Form/ParticipantType.php

@@ -322,7 +322,7 @@ class ParticipantType extends AbstractType
                 return;
             }
 
-            $transportationId = $data['transportationServiceTo'];
+            $transportationId = $data['transportationServiceTo'] ?? null;
             $transportation = $options['travel']->pickups[$transportationId] ?? null;
 
             if (null !== $transportation && 'PKW' === $transportation->subType) {