Playing a deleted instrument with a MIDI keyboard causes the program to crash

[CreeperPookie]

Describe the bug
I recently got a small MIDI keyboard for Note Block Studio, and it's been really nice! However, when doing some testing with custom Minecraft instruments, I found that removing an instrument assigned to the keyboard causes the program to crash (which makes sense)

To Reproduce
Steps to reproduce the behavior:

1. Connect a MIDI keyboard to your computer (Some Android phones can also serve as a MIDI keyboard through apps if set correctly)
2. Create a custom instrument and assign it to the MIDI device and make sure it works with the keyboard
3. Delete the custom instrument without reassigning the assigned instrument in the MIDI devices screen
4. See crash

Expected behavior
While I know, this is an incredibly niche situation I would expect OpenNBS to change the assigned instrument to a fallback on disconnect (say harp or pling), so if a key is pressed after deletion, the program can continue normally

Screenshots
Error screenshot: