If the removal of an instrument is undone, the instrument will not produce any audio.

When an instrument is removed, and then re-added using undo, the instrument will not be present in the mixer, and will not produce any audio until the file is reopened.

Steps to reproduce:
- Remove an instrument from any score. (it also works when removing multiple instruments).
- Undo the removal (I used Ctrl+z).
- Create some audio with that instrument. This can be done by:
- Selecting a note
- Adding a note
- Hitting play
- Notice that there is no audio.
- Open the mixer
- Notice that the instrument isn't present there.

When the score is closed and reopened, the audio returns to normal and the instruments appear in the mixer again.

I have been able to reproduce this on all of my PCs with MuseScore installed.