By passing NULL to `g_signal_new` instead of a marshaller, GLib will
actually internally optimize the signal (if the marshaller is available
in GLib itself) by also setting the valist marshaller. This makes the
signal emission a bit more performant than the regular marshalling,
which still needs to box into `GValue` and call libffi in case of a
generic marshaller.
Note that for custom marshallers, one would use
`g_signal_set_va_marshaller()` with the valist marshaller instead.
The issue happens when the structure is printed by the logging
subsystem: the object is included in the log, and this will cause the
full object printout to be done there. However, after dispose, the queue
was already cleared, so the access to it (to print the object) would
assert, as the queue was already freed. The patch changes it so that the
queue is merely empty, and only freed in _finalize.
https://bugzilla.gnome.org/show_bug.cgi?id=776293
