I believe there is a small bug in the file "Mission.lua":
It is missing the declaration of the "Game" variable (Game = import("Game")).
This will only become obvious if you create a new mission (Mission.New()) and don't pass a valid location to the method. In that case the fallback "newMission.location = Game.system.path" is triggered (line 251), but the variable "Game" has not been declared, causing a game crash.