An interesting observation while trying to reproduce this: it only happens if you "hot-plug" the second monitor on an already started system. It doesn't happen if the system had the external monitor connected when it booted. That is:
Scenario A
1- start a laptop with no external monitor connected
2- plug in the external monitor
3- start checkbox-gui
4- press the monitor switching key several times
5- BOOM CRASH
Scenario B
1- start a laptop with external monitor already connected
2- start checkbox-gui
4- press monitor switching key several times
5- no crash
An interesting observation while trying to reproduce this: it only happens if you "hot-plug" the second monitor on an already started system. It doesn't happen if the system had the external monitor connected when it booted. That is:
Scenario A
1- start a laptop with no external monitor connected
2- plug in the external monitor
3- start checkbox-gui
4- press the monitor switching key several times
5- BOOM CRASH
Scenario B
1- start a laptop with external monitor already connected
2- start checkbox-gui
4- press monitor switching key several times
5- no crash