Fix linting issues

lib/drawer/class_list_item.dart

@@ -25,9 +25,8 @@ class _ClassListItemState extends State<ClassListItem> {
       widget.appContext.profile.value!.selectedClasses;
   bool get isSelected => selectedClasses.contains(widget.cls.id);
 
-  List<Lecture>? get lectures => widget.appContext.lectures == null
-      ? null
-      : widget.appContext.lectures!.where(isRelevantLecture).toList();
+  List<Lecture>? get lectures =>
+      widget.appContext.lectures?.where(isRelevantLecture).toList();
 
   bool isRelevantLecture(Lecture lecture) {
     return lecture.code.startsWith(widget.cls.code);

lib/timetable/lesson_chip.dart

@@ -16,10 +16,10 @@ class LessonChip extends StatelessWidget {
     return pressable ? theme.primaryColor : theme.iconTheme.color!;
   }
 
-  static MaterialStateColor getTextColor(BuildContext context) =>
-      MaterialStateColor.resolveWith((states) => getTextColorOfState(
+  static WidgetStateColor getTextColor(BuildContext context) =>
+      WidgetStateColor.resolveWith((states) => getTextColorOfState(
             context,
-            pressable: !states.contains(MaterialState.disabled),
+            pressable: !states.contains(WidgetState.disabled),
           ));
 
   @override

lib/timetable/period_background.dart

@@ -18,7 +18,7 @@ class PeriodBackground extends StatelessWidget {
       offset: Offset(0, start * LessonWidget.zoom),
       child: Container(
         decoration: BoxDecoration(
-          color: Theme.of(context).colorScheme.surfaceVariant,
+          color: Theme.of(context).colorScheme.surfaceContainerHighest,
         ),
         height: (end - start) * LessonWidget.zoom,
       ),

lib/toolbar/toolbar.dart

@@ -16,7 +16,7 @@ class _ToolbarState extends State<Toolbar> {
   @override
   Widget build(BuildContext context) {
     return Container(
-      color: Theme.of(context).colorScheme.background,
+      color: Theme.of(context).colorScheme.surface,
       child: Row(
         mainAxisAlignment: MainAxisAlignment.center,
         children: [

lib/untis/element.dart

@@ -10,7 +10,7 @@ mixin UntisMixin {
   String toString() => name;
 
   @override
-  bool operator ==(dynamic other) {
+  bool operator ==(Object other) {
     if (other is! UntisElement) return false;
 
     return other.hashCode == hashCode;