Don't update next stack if there is no next stack
Don't update next stack if there is no next stack

Not all extruder stacks have a next stack. Well currently every extruder stack should have a next stack, but the code is built around the possibility that there isn't a next stack elsewhere too.

Contributes to issue CURA-4121.

Curahelper/Cura,Curahelper/Cura