g_dbus_connection_call_done() has this logic:

  reply = g_dbus_connection_send_message_with_reply_finish (connection,
                                                            result,
                                                            &error);

...

  if (reply != NULL)
    state->value = decode_method_reply (reply,
                                        state->method_name,
                                        state->reply_type,
                                        &state->fd_list,
                                        &error);

  simple = state->simple;
  if (error != NULL)
    {
      g_simple_async_result_take_error (state->simple, error);
      g_simple_async_result_complete (state->simple);
      call_state_free (state);
    }
  else
    {
      g_simple_async_result_set_op_res_gpointer (state->simple,
        state, (GDestroyNotify) call_state_free);
      g_simple_async_result_complete (state->simple);
      g_object_unref (reply);
    }

Which is clearly capable of having reply != NULL with error set.

In the (error != NULL) case reply isn't freed, and it leaks.