Created attachment 251769 [details] [review]
Proposed patch

I observed this with the "name" argument, but looks like it can also happen with the "user_data" argument in these callback functions:

void  (*GBusNameAppearedCallback)  (GDBusConnection *connection,
                                    const gchar *name,
                                    const gchar *name_owner,
                                    gpointer user_data);

void  (*GBusNameVanishedCallback)  (GDBusConnection *connection,
                                    const gchar *name,
                                    gpointer user_data);

The actually_do_call() function should reference the "Client" struct before invoking either of these callbacks to keep them alive for the duration of the callback.