I've been having this same bug ever since the last Apple Security Update. If I open a message window, or if someone messages me, or I go to preferences to look at message styles, the program crashes. I tried moving all files and re-installing the program, starting from scratch, and the same thing happened, both in 0.77 and in 0.8b10. I know it's not your program since apparently nobody else is having this problem, so I think some file that it accesses somewhere to render message styles got corrupted.
Any ideas on this?