Unsubscribes from debug vis handle when timeline is stopped (#2214)

# Description
This PR fixes a bug where old callbacks for sensors were being called
after the scene had been cleared, resulting in exceptions and a crash of
extensions built off of Isaac Lab.

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

Author: jsmith-bdai

source/isaaclab/isaaclab/assets/asset_base.py

@@ -269,3 +269,6 @@ def _initialize_callback(self, event):
     def _invalidate_initialize_callback(self, event):
         """Invalidates the scene elements."""
         self._is_initialized = False
+        if self._debug_vis_handle is not None:
+            self._debug_vis_handle.unsubscribe()
+            self._debug_vis_handle = None

source/isaaclab/isaaclab/sensors/sensor_base.py

@@ -276,6 +276,9 @@ def _initialize_callback(self, event):
     def _invalidate_initialize_callback(self, event):
         """Invalidates the scene elements."""
         self._is_initialized = False
+        if self._debug_vis_handle is not None:
+            self._debug_vis_handle.unsubscribe()
+            self._debug_vis_handle = None
 
     """
     Helper functions.