Hi thank you for buying the game and for your report. It's already on my list to fix as I found the bug myself during gameplay. The bug is not limited to Level 7 of the game where you found it and it can happen at any point. Unfortunately the game requires to be restarted if you cannot quit the game via the RESTORE key.
UPDATE: I am just going through some final testing, but the bug should be fixed in the next release R4.3.