There's a couple of places in GDBus where it's a programming error (e.g. we'll assert or spew via e.g. g_warning()) to use the API on a closed connection. This approach can never work since a GDBusConnection can be closed at any point in time outside of programmer control.

Just change the code to return a run-time error (e.g. return G_IO_ERROR_CLOSED when sending messages, invoking methods) or silently accept the request (e.g. exporting objects, registering for signals) without doing anything.

Note that a GDBusConnection object is always useless after being closed - e.g. there's no way to "reopen" a connection - the user will have to create a new object and use that instead.