getAll method Null safety

Implementation

Future<Map<String, Product>> getAll(final Iterable<String> barcodes) async {
  final LazyBox<Product> box = _getBox();
  final Map<String, Product> result = <String, Product>{};
  for (final String barcode in barcodes) {
    final Product? product = await box.get(barcode);
    if (product != null) {
      result[barcode] = product;
    }
  }
  return result;
}