menubar not showing menu items

[xiezhuojin]

Checklist

• [X] I have searched for similar issues.
• [X] For Python issues, I have tested with the latest development wheel.
• [X] I have checked the release documentation and the latest documentation (for master branch).

Describe the issue

menubar not showing menu items.

Steps to reproduce the bug

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

- 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