#624 Assertion in persistence

[Christa Runge]

the following issue was seen at r3918:
After test / drill see the Drill Finished Screen as expected.
Press the Done button.
Expected behavior: see the list of databases
Observed behavior: the screen freezes; XCode debugger stops at assertion (PersistenceHandler line 79) in method deregisterObject because the persistence object stack is inconsistent.
Object to be registered is a DrillNavigationController as expected, but last object popped from the stack is a CatTableViewController (unexpectedly).

Configuration:
A local build at r3918, with VERIFY_DRILL_NUMBERS active, linked with SDK iOS 11.4.
Executed on iPad Air device (iOS 11.2.2), connected to XCode debugger.

Note: No data corruption. After stopping execution in the debugger, the application could be started again without problems.
Note: low prio because we expect this hard to reproduce and track down.