To fix the issue, I introduced a test whether the game logic object has
created single games (by trying to access one). Only if the game logic
object has single games, stop game and delete single games is called.

I have also confirmed by manual testing that the game no longer crashes
when (re)starting a game after the first one.

The test is not as unit-test-y as I would like: Several QWARN are
issued. Furthermore, there is another option to improve usability:

The last piece is also placed ontop of an existing piece, triggering a
game over. However, this final piece is still invisible.