Prevent crashes due to circular activity calls

[paetz]

To enable jumps from tourbook entry to e.g. route, we need some crash prevention.
Scenario:

1. Go to ascent via tourbook
2. From there go to the according route (to be implemented)
3. Delete the according ascent from the route view
4. Navigate back (via back button) to the ascent view --> This is gonna crash since ascent is not available anymore.

Solution would be to check for existing ascent on entering the ascent view. If not, toast an error and go back to database view or (if possible) to the previous view in the calling history.