Add annotation (allow-none) to the parameter error.
https://bugzilla.gnome.org/show_bug.cgi?id=677952
* @connection: The #GDBusConnection emitting the signal.
* @remote_peer_vanished: %TRUE if @connection is closed because the
* remote peer closed its end of the connection.
- * @error: A #GError with more details about the event or %NULL.
+ * @error: (allow-none): A #GError with more details about the event or %NULL.
*
* Emitted when the connection is closed.
*