better loading indicators for timetables, talk and files

lib/view/pages/files/files.dart

@@ -164,9 +164,9 @@ class _FilesViewState extends State<_FilesView> {
         child: const Icon(Icons.add),
       ),
       body: LoadableStateConsumer<FilesBloc, FilesState>(
+        isReady: (state) => state.listing != null,
         child: (state, _) {
-          final listing = state.listing;
-          if (listing == null) return const SizedBox.shrink();
+          final listing = state.listing!;
           if (listing.files.isEmpty) {
             return const PlaceholderView(icon: Icons.folder_off_rounded, text: 'Der Ordner ist leer');
           }