menubar not showing menu items

### 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

menubar not showing menu items.

### Steps to reproduce the bug

```python
import open3d as o3d
import open3d.visualization.gui as gui
import open3d.visualization.rendering as rendering

app = gui.Application.instance
app.initialize()

menu = gui.Menu()
menu.add_item("a", 1)
menu.add_item("b", 2)
menu.add_menu("test", gui.Menu())
app.menubar = menu

window = app.create_window("test")
window.size_to_fit()
scene = gui.SceneWidget()
scene.scene = rendering.Open3DScene(window.renderer)
window.add_child(scene)

app.run()
```

### Error message

_No response_

### Expected behavior

The menu item should show.

### Open3D, Python and System information

```markdown
- Operating system: Windows 10 64-bit
- Python version: Python 3.9.13
- Open3D version: 0.15.1
- System architecture: x86
- Is this a remote workstation?: no
- How did you install Open3D?: pip
```

### Additional information

_No response_