Cannot decode server response to json

[khangahs]

I'm trying to do request for POST with GetConnect and it return a Cannot decode server response to json. My backend is application/json. Also, I cannot print out the res.bodyString, which poses a very bothersome problem.

Future<List<MainItem>?> addItem({
    Key? key,
    @required String? name,
    @required String? description,
    @required String? userId,
    @required String? imageUrl,
    @required String? streetAddress,
    @required String? locationName,
    @required String? locationDescription,
    @required String? city,
    @required String? itemUuid,
    @required int? zipCode,
    @required int? itemTypeId,
    @required int? stateId,
    @required int? countryId,
  }) async {
    var mainItem = MainItem(
      name: name,
      description: description,
      itemTypeId: itemTypeId,
      userId: userId,
      itemUuid: itemUuid,
      inventory: Inventory(
          inventoryUuid: '',
          box: 0,
          quantity: 0,
          itemInBox: 0,
          inStorageItem: 0,
          inStoragePrice: 0,
          pricePerItem: 0),
      //
      image: ItemImage(imageUrl: imageUrl),
      location: InventoryLocation(
          name: locationName,
          description: locationDescription,
          address: Address(
              countryId: countryId,
              stateId: stateId,
              streetAddress: streetAddress,
              city: city,
              zipCode: zipCode)),
    ).toJson();
    final res = await post('/item/$userId', mainItem);
    if (res.hasError) {
      return Future.error(res.statusText!);
    }
 return (res.body as List).map((e) => MainItem.fromJson(e)).toList();
  }

We discuss also in StackOverFlow:
https://stackoverflow.com/questions/67634010/cannot-decode-server-response-to-json-flutter?noredirect=1#comment119630408_67634010