Make sure that the preferred material name variable exists
Make sure that the preferred material name variable exists

Another small bug I found when looking through this MachineManager code. My IDE was complaining that the preferred_material_name was not always initialised and that preferred_material was an unused variable.

Doesn't contribute to issue CURA-3803.

Curahelper/Cura,Curahelper/Cura,fieldOfView/Cura,ynotstartups/Wanhao,ynotstartups/Wanhao,fieldOfView/Cura