asyncLoad method Null safety

Implementation

@override
Future<String?> asyncLoad() async {
  try {
    // we start from page 1
    ProductList productList = productQuery.getProductList();
    bool first = true;
    do {
      // we try to get the locally saved data for the current page
      await DaoProductList(localDatabase).get(productList);
      if (productList.barcodes.isEmpty) {
        // we found nothing
        if (first) {
          // we save an empty list
          partialProductList.add(productList);
        }
        // that's it, we've just loaded all the non-empty pages we could
        return null;
      }
      // we found something: let's add it to the partial product list
      partialProductList.add(productList);
      // and try again with the next page
      productQuery.toNextPage();
      productList = productQuery.getProductList();
      first = false;
    } while (true);
  } catch (e) {
    return e.toString();
  }
}