Fix 'AttributeError' (defensive coding).

should fix sentry#CURA-1D2

cura/Settings/MachineManager.py

@@ -398,7 +398,7 @@ class MachineManager(QObject):
                 self.setVariantByName(extruder.getMetaDataEntry("position"), machine_node.preferred_variant_name)
                 variant_node = machine_node.variants.get(machine_node.preferred_variant_name)
 
-            material_node = variant_node.materials.get(extruder.material.getMetaDataEntry("base_file"))
+            material_node = variant_node.materials.get(extruder.material.getMetaDataEntry("base_file")) if variant_node else None
             if material_node is None:
                 Logger.log("w", "An extruder has an unknown material, switching it to the preferred material")
                 if not self.setMaterialById(extruder.getMetaDataEntry("position"), machine_node.preferred_material):