I didn't have a deep look, but it may come from here :
https://github.com/chbergmann/CurvedSha ... ray.py#L41
You are storing a pointer to the FeaturePython object.
That looks easier, but since it is not a standard python type, JSON is not able to save / restore it.
You can implement your own save / restore mechanism in setState() / getState().
But I think a better way is to transfer it to your custom Proxy methods :
All the special methods of a Proxy object (execute(), onChanged(), onBeforeChange(), onDocumentRestored()) get the FeaturePython object as first argument, so you can probably transfer this reference to your other custom methods, without having to save a reference in self.obj.