Hive not storing Object when App restarts???

Hello, in my Flutter App using Hive is all running correct till I restart the App. Then the box is empty and the Object is not stored?
Till I restart the App again, all is working fine. The Object is generated with the build_runner

```
@HiveType(typeId: 0)
class Favorites extends HiveObject {
@HiveField(0)
List mediaItems = [];
}

final document = await getApplicationDocumentsDirectory();
Hive
..init(document.path)
..registerAdapter(FavoritesAdapter())

await Hive.openBox('favorites');
.......

Box box = Hive.box('favorites');
Favorites fav = box.getAt(0) ?? Favorites();
fav.mediaItems.insert(0, playlist[i]);
```

The data are stored and when I restart the App the Box is empty???
Thanks for helping!