fixed issues

refilc/lib/helpers/notification_helper.dart

@@ -22,7 +22,7 @@ import 'package:intl/intl.dart';
 import 'package:refilc_kreta_api/models/message.dart';
 
 // if you want to add a new category, also add it to the DB or else the app will probably crash
-enum LastSeenCategory { 
+enum LastSeenCategory {
   grade,
   surprisegrade,
   absence,
@@ -655,7 +655,7 @@ nem lesz tőle használhatatlan az app, de kikommenteltem, mert még a végén k
                         lesson.lessonIndex,
                         lesson.name,
                         dayTitle(lesson.date),
-                        lesson.substituteTeacher!.isRenamed
+                        (lesson.substituteTeacher?.isRenamed ?? false)
                             ? lesson.substituteTeacher!.renamedTo!
                             : lesson.substituteTeacher!.name
                       ],