How to pass null property to register_object

Question

When calling registry_object, the following does not work:

use gio::DBusConnection;

let dbus_connection: DBusConnection = ...
dbus_connection.register_object(
  path,
  self.introspection_data.unwrap(),
  self.handle_method_call,
  None, //get_property
  None, //set_property
).unwrap();

Both arguments get_property and set_property want a fn but I have no idea how to map this function so that I would be null - similar to C. Setting None does not work :(