Fix adding materials in UFP

CURA-5555

Need to use getContainer() for a MaterialNode.

plugins/UFPWriter/UFPWriter.py

@@ -90,7 +90,7 @@ class UFPWriter(MeshWriter):
                 Logger.log("e", "Cannot find material container with root id [%s]", material_root_id)
                 return False
 
-            material_container = material_group.root_material_node
+            material_container = material_group.root_material_node.getContainer()
             try:
                 serialized_material = material_container.serialize()
             except NotImplementedError: