Explicitly return None if no global container stack
Explicitly return None if no global container stack

That's what it does. This is more clear than doing 'pass' or something.

Contributes to issues CURA-340 and CURA-1278.

fieldOfView/Cura,totalretribution/Cura,ynotstartups/Wanhao,Curahelper/Cura,senttech/Cura,fieldOfView/Cura,Curahelper/Cura,hmflash/Cura,ynotstartups/Wanhao,senttech/Cura,totalretribution/Cura,hmflash/Cura