o3d.visualization.webrtc_server.enable_webrtc()

### 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).

### Describe the issue

why can occure this?

### Steps to reproduce the bug

```python
import open3d as o3d
o3d.visualization.webrtc_server.enable_webrtc()
```

### Error message

air

### Expected behavior

air

### Open3D, Python and System information

```markdown
Operating system: Windows 10 64-bit
Python version: Python 3.7 / output from `import sys; print(sys.version)`
Open3D version: 0.15 / 0.14 /output from python: `print(open3d.__version__)`
System architecture: x86
Is this a remote workstation?: no
How did you install Open3D?: conda
```

### Additional information

air