Fix checking if an extruder train already exists
Fix checking if an extruder train already exists

Contributes to issues CURA-340 and CURA-1278.

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