I think the problem here is that you have your adapter read method in the wrong order. Can you confirm if the following fixes it?

CartHive read(BinaryReader reader) {
    return CartHive(
      (reader.readMap()).cast<String, int>(), // this should be the second arg
      (reader.readMap()).cast<String, String>(), // this should be the first arg
      (reader.readMap()).cast<String, int>(),
    );