How can I get the viewing angle from SceneWidget?

### 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

I would like to get the camera viewing angle of a pointcloud which is displayed using `SceneWidget()` whose scene is set like this:

```
self.window = gui.Application.instance.create_window("Open3D", width, height)
self._scene = gui.SceneWidget()
self._scene.scene = rendering.Open3DScene(self.window.renderer)
```

If I use `o3d.visualization.Visualizer()`, I can get it using `get_view_control()`, but cannot figure out how to get it using `SceneWidget`.

Some links mentioned I can initialize the `SceneWidget()` using the `Visualizer()` but I cannot get it to work.