Do any other games have this problem for you? As I know that a number of games when detecting a change in display like that will freak out. You might try, setting your primary display as the external, closing your laptops lid and then starting FE.

Hmm fair enough, if you do mostly just use the external display you might also try turning off your display on the laptop and putting it in projector mode or w/e its called. This is often done through a fn key and a f1-12 key, sometimes its a different combination. This would allow you however, to open or close your lid, supposedly without consequence, reality is often different however. Programs often detect the change in state caused by the lid closing, even if the OS is told to do nothing there is often a pause and switch for the GPU. This causes games to crash or hang often.