I have Handcent installed so I can send MMS messages only because the default app is still shrinking images. No issues here. However, I have all the notifications within Handcent disabled so it uses the default messaging notifications.
You could try the same, see what happens. If it stops crashing at that point ,then it has something to do with the way Handcent is intercepting notifications. In any case, it would probably have to have an update from Handcent to fix.
worst case scenario hit up the google play store and read the reviews on the handcent app itself..see if others are complaining about this.