FIXED TabGroupActivity crashes

*Touches Wood* this appears to have fixed the crashes occurring when the
stack goes sour. The problem actually occurs due to bad implementation
of the TabGroupActivity stack, which should be fully implemented, but is
only partially implemented throughout the app. This fix, though hacky,
checks for null pointer exceptions before trying anything, a check which
should have already been present anyway.

The implementation of this fix is not final with this update, and the
implementation of the TabGroupActivity class should be properly
completed in a future commit, but this fix brings it down from "That's
crashing an awful lot, isn't it?" to "Oh, well okay, this [occasionally]
isn't quite the tab I was hoping to get to when I pressed back, but it's
at least, kind of.. close... I suppose?"