MacOS (ARM): crash when changing image while playing MIDI

[bboettcher3]

Reported from a user. Still being investigated, can't seem to reproduce on Windows..

[atsushieno]

Maybe it's not surprising, I hit the crash near TODO: fix bug where gNote is null here in Debug:

(M2 mac, Standalone app, build at a20ba72.)

[bboettcher3]

May be a duplicate of #58 , if not I can reopen