decode method Null safety

Implementation

@override
void decode(KeyedArchive object) {
  super.decode(object);

  paths = {};
  object.forEach((key, dynamic value) {
    if (value is! KeyedArchive) {
      throw ArgumentError(
        "Invalid specification. Callback contains non-object value.",
      );
    }
    paths![key] = value.decodeObject(key, () => APIPath())!;
  });
}