If we have no active machine, return None in getEngineCommand
If we have no active machine, return None in getEngineCommand

This fixes a race condition on Windows where setting the active machine
takes too long

totalretribution/Cura,bq/Ultimaker-Cura,markwal/Cura,hmflash/Cura,senttech/Cura,Curahelper/Cura,senttech/Cura,ynotstartups/Wanhao,ad1217/Cura,fieldOfView/Cura,ad1217/Cura,markwal/Cura,bq/Ultimaker-Cura,Curahelper/Cura,fieldOfView/Cura,hmflash/Cura,ynotstartups/Wanhao,totalretribution/Cura