prevent common "change" tiles in timetable, more robust parsing

lib/view/pages/timetable/timetable.dart

@@ -1,6 +1,6 @@
-
 import 'dart:async';
 
+import 'package:collection/collection.dart';
 import 'package:flutter/material.dart';
 import '../../../extensions/dateTime.dart';
 import 'package:provider/provider.dart';
@@ -302,23 +302,25 @@ class _TimetableState extends State<Timetable> {
         var startTime = _parseWebuntisTimestamp(element.date, element.startTime);
         var endTime = _parseWebuntisTimestamp(element.date, element.endTime);
 
-        var subject = subjects.result.firstWhere((subject) => subject.id == element.su[0].id);
-        var subjectName = {
+        var subject = subjects.result.firstWhereOrNull((subject) => subject.id == element.su.first.id);
+        var subjectName = 'Unbekannt';
+        if(subject != null) {
+          subjectName = {
             TimetableNameMode.name: subject.name,
             TimetableNameMode.longName: subject.longName,
             TimetableNameMode.alternateName: subject.alternateName,
-        }[settings.val().timetableSettings.timetableNameMode];
-
+          }[settings.val().timetableSettings.timetableNameMode]!;
+        }
 
         return Appointment(
           id: ArbitraryAppointment(webuntis: element),
           startTime: startTime,
           endTime: endTime,
-          subject: subjectName!,
+          subject: subjectName,
           location: ''
-              '${rooms.result.firstWhere((room) => room.id == element.ro[0].id).name}'
+              '${rooms.result.firstWhereOrNull((room) => room.id == element.ro.firstOrNull?.id)?.name ?? 'Unbekannt'}'
               '\n'
-              '${element.te.first.longname}',
+              '${element.te.firstOrNull?.longname ?? 'Unbekannt'}',
           notes: element.activityType,
           color: _getEventColor(element, startTime, endTime),
         );
@@ -331,7 +333,7 @@ class _TimetableState extends State<Timetable> {
           subject: 'Änderung',
           notes: element.info,
           location: 'Unbekannt',
-          color: endTime.isBefore(DateTime.now()) ? Theme.of(context).primaryColor.withAlpha(100) : Theme.of(context).primaryColor,
+          color: const Color(0xff404040).withAlpha(endTime.isBefore(DateTime.now()) ? 100 : 255),
           startTimeZone: '',
           endTimeZone: '',
         );