I've been having this problem, too. For a few versions now, I wanna say, but I run the betas, too, so...
It actually throws TWO seemingly-identical error dialogs when I shut C2 down. No idea why. It doesn't seem to happen if I open C2 for just a short time and close it again. But if I leave it open for hours (and I do, a lot), it's 99.999999% likely to crash (seemingly twice).
Wish I had more info to share (debug mode? hidden log file? happy to send whatever)