fix dart analysis issues

lib/presentation/views/main_menu/custom_navigation_bar.dart

@@ -141,7 +141,7 @@ class _CustomNavigationBarState extends State<CustomNavigationBar>
   }
 
   /// Returns the title of the current tab based on [currentIndex].
-  String _currentTabTitle(context) {
+  String _currentTabTitle(BuildContext context) {
     final loc = AppLocalizations.of(context);
     switch (currentIndex) {
       case 0:

lib/presentation/views/main_menu/group_view/create_group_view.dart

@@ -147,7 +147,7 @@ class _CreateGroupViewState extends State<CreateGroupView> {
                         (selectedPlayers.length < 2))
                     ? null
                     : () async {
-                        Group? updatedGroup = null;
+                        Group? updatedGroup;
                         bool successfullNameChange = true;
                         bool successfullMemberChange = true;
                         late bool success;

lib/presentation/widgets/app_skeleton.dart

@@ -47,10 +47,7 @@ class _AppSkeletonState extends State<AppSkeleton> {
             : (Widget? currentChild, List<Widget> previousChildren) {
                 return Stack(
                   alignment: Alignment.topCenter,
-                  children: [
-                    ...previousChildren,
-                    if (currentChild != null) currentChild,
-                  ],
+                  children: [...previousChildren, ?currentChild],
                 );
               },
       ),