Thanks a lot, that was very enlightening ;)
I'm unable to catch the exception output in the simulator - I'd probably need the serial debug port for that.
Anyway, your explanation sounds plausible and I modified the onExit callback function accordingly.

Regarding the "null"-ing -  I had thought that those callbacks would be cleared by the system when changing pages - just like everything else is cleared when changing activities.
Apparently that assumption was wrong - I just verified it checking for the page's .onExit value from another page when not "null"-ing it.
That's good to know!
I hope for your kitchen floor (and your spouse, of course) that your Post-it note management is just as thorough... :D

Sebastian