Don't crash if no screens available (no access to the window system)
Don't crash if no screens available (no access to the window system)

Fixes a crash when running gengal on OS X in a ssh session when not
logged in through the window system at the same time.

Change-Id: I6a47666aeac43fc140f52ae2d62b36eadc976246

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core