Fix crash when enabling setting visibility for adhesion_extruder_nr or support_extruder_nr
Fix crash when enabling setting visibility for adhesion_extruder_nr or support_extruder_nr

The fix is likely not what was intended by the author of the offending code (and does not seem to be functional), but at least it fixes the hard crash outlined in CURA-1666

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