Fix bug where editing a single material didn't quite work.

src/blenderbim/blenderbim/bim/module/material/operator.py

@@ -473,7 +473,6 @@ class EditAssignedMaterial(bpy.types.Operator):
         obj = bpy.data.objects.get(self.obj) if self.obj else bpy.context.active_object
         props = obj.BIMObjectMaterialProperties
         product_data = Data.products[obj.BIMObjectProperties.ifc_definition_id]
-        material_set = self.file.by_id(self.material_set)
 
         if product_data["type"] == "IfcMaterial":
             bpy.ops.bim.unassign_material(obj=obj.name)
@@ -482,6 +481,8 @@ class EditAssignedMaterial(bpy.types.Operator):
             bpy.ops.bim.disable_editing_assigned_material(obj=obj.name)
             return {"FINISHED"}
 
+        material_set = self.file.by_id(self.material_set)
+
         attributes = {}
         for attribute in props.material_set_attributes:
             attributes[attribute.name] = None if attribute.is_null else attribute.string_value

src/blenderbim/blenderbim/bim/module/material/ui.py

@@ -97,6 +97,8 @@ class BIM_PT_object_material(Panel):
                 self.material_set_data = Data.lists[self.material_set_id]
                 self.set_items = self.material_set_data["Materials"] or []
                 self.set_item_name = "list_item"
+            else:
+                self.material_set_id = 0
             return self.draw_material_ui()
 
         row = self.layout.row(align=True)