[MU4 Issue] When opening score that contains no instruments, "Loading audio samples" stays forever

Describe the bug
If you open a score, the "Loading audio samples" message appears in the Status Bar. However, if you open a score that contains no instruments, the loading message stays forever.

To Reproduce
Steps to reproduce the behavior:

1. Create a score
2. Remove all instruments using the instruments panel
3. save and close the score
4. reopen the score
   The "Loading audio samples" message appears and stays forever.

Expected behavior
If there are no instruments, nothing needs to be loaded, so the loader shouldn't appear at all. However, in practice, the metronome track would still need to be loaded, so the loader should appear only for that metronome track and disappear after that.

Platform information

• OS: reported on macOS