A valid version of iCal was not selected . . . message stuck on screen.

Just updated from Lion to Mountain Lion. A popup message saying "A valid version of iCal was not selected. The icon could not be updated" is stuck on my screen. Selecting OK, hitting O, or hitting enter does nothing. ESC does nothing. Everything seems to be working okay. This happened when I first installed the update and rebooting helped. But it just popped up again. I don't want to have to keep rebooting. Does anyone have any ideas about this?

Thanks,

Tery