# bookletRepository method Null safety

1. @override

BookletRepository? bookletRepository(

1. String? appId

)
override

## Implementation

    @override
    BookletRepository? bookletRepository(String? appId) {
      if ((appId != null) && (_bookletRepository[appId] == null)) {
        _bookletRepository[appId] = BookletCache(BookletFirestore(
            () => appRepository()!.getSubCollection(appId, 'booklet'), appId));
      }
      return _bookletRepository[appId];
    }