This dialog box is supposed to disappear when user presses Esc once. But if he accidentally hits the button twice, browser crashes.
Surely Safari has evolved from 3.1 era and AppleWin port is gone since this issue seems to be reported on Windows. I think if we had this issue, it would've been reported via Apple Feedback Assistant since seems more like Safari issue than WebKit. Marking this as 'RESOLVED WONTFIX'. Please reopen if it is still reproducible.