What is the warining message "Geometry geometry.__fast__ is not in the scene graph"

### 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

Dear all.

First of all, I'm sorry for my bad English.

I trying to learn point cloud data sampling using python language.
And I display point cloud using SceneWidget for the test enviroments.

This is one issue.

If I display the original data(It means using loaded point cloud data at the first time), there is no problem.

However, after applying an algorithm(eg. voxel down sampling) to the point cloud data, when I output the changed data to a SceneWidget and dragging display, the screen flickers and "[Open3D WARNING] Geometry geometry is not in the scene graph" message has printed on python terminal.

What means of "[Open3D WARNING] Geometry geometry is not in the scene graph" message and Can I know that which case the message is displayed?

Thank you.