simplified code across the app and consolidated duplicated helpers

lib/view/pages/timetable/details/delete_custom_event.dart

@@ -8,12 +8,13 @@ import '../../../../state/app/modules/timetable/bloc/timetable_bloc.dart';
 import '../../../../widget/confirm_dialog.dart';
 import '../../../../widget/demo_restricted.dart';
 
-Completer<void> showDeleteCustomEventDialog(
+/// Completes once the event was deleted; never when the dialog is dismissed.
+Future<void> showDeleteCustomEventDialog(
   BuildContext context,
   CustomTimetableEvent event,
 ) {
   final completer = Completer<void>();
-  if (guardDemoAction(context)) return completer;
+  if (guardDemoAction(context)) return completer.future;
   final bloc = context.read<TimetableBloc>();
   ConfirmDialog(
     title: 'Termin löschen',
@@ -25,5 +26,5 @@ Completer<void> showDeleteCustomEventDialog(
       completer.complete();
     },
   ).asDialog(context);
-  return completer;
+  return completer.future;
 }