How to wrap Scene Widget in WidgetProxy / proper way to remove SceneWidget from window

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Hey together,
I currently work on a project where I add SceneWidgets to a window. In some scenarios I want to remove a specific Scene Widget from the window.
For other Widgets this can be easily done using a WidgetProxy but when I wrap a SceneWidget into a WidgetProxy, the Scene widget seems to work normal first and I can also navigate using the scroll wheel and the right mouse button but it becomes unusable as soon as I leftclick the SceneWidget (it looks like the whole scene disappears and only the background is left).
Before left clicking the widget:

After left clicking the widget:

Anybody knows what is going on here and how to fix that?

If a WidgetProxy does not work for this: What is the proper way to remove a SceneWidget?

Any help is greatly appreciated!

Thank you in advance!