Combobox subscribe_item_changed_fn event is not called

i am using this combobox example

and here is my code:

    def _simulator_type_selection_frame(self):
         
        simulation_type_list = [option.name for option in SimulatorType]

        with ui.CollapsableFrame("Simulator type: map generator OR randomized data", collapsed=False):
            with ui.HStack():
                ui.Label("Simulator type", name="label", width=WidgetWindow.LABEL_PADDING - 30)
                # Create a combo box for the enum, setting the current value from Interface
                selection_box = ui.ComboBox(self._delegate._simulator_type.value, *simulation_type_list)


                def rebuild_window(item_model: ui.AbstractItemModel, item: ui.AbstractItem):
                    print("enum value changed")
              

                
                self._delegate.set_simulator_type_selection(selection_box.model)
                selection_box.model.subscribe_item_changed_fn(rebuild_window)

the rebuild_window function is never called.

Thank you