PoseGraph in CPP example

### 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

I have tried to run the PoseGraph example in CPP.
PoseGraph.exe test_pose_graph.json
A warning occured:
[Open3D WARNING] Invalid Posegraph - graph is not connected.

### Steps to reproduce the bug

```python
PoseGraph test_pose_graph.json
```

### Error message

[Open3D WARNING] Invalid Posegraph - graph is not connected.

### Expected behavior

_No response_

### Open3D, Python and System information

```markdown
- Operating system: (Windows 10 64-bit)
- Open3D version: (0.14)
- System type: (x84 )
- Is this remote workstation?: no
- How did you install Open3D?: (build from source)
- Compiler version (VS2019)
```

### Additional information

No code modifyied in this example.