Complicated bug report

I found a complicated bug during testing, which is unlikely to manifest itself in a normal work process.
It's not a big deal. It doesn't affect the functionality, just the interface. Saving and opening the session again fixes it.
I'm reporting it anyway so it's known.

Playback:
- Deletes a bus that has tracks associated with it.
- Deletes a track that was associated with the bus.
- The track doesn't disappear from the mixer.