Clear node cache when switching to/from sandbox

lib/widgets/map_view.dart

@@ -158,7 +158,7 @@ class MapViewState extends State<MapView> {
       ScaffoldMessenger.of(context).showSnackBar(
         SnackBar(
           content: Text(message),
-          duration: const Duration(seconds: 2),
+          duration: const Duration(seconds: 4),
           behavior: SnackBarBehavior.floating,
         ),
       );