gtk_toolbar_finalize() has code to remove and destroy all its internal and external children. The external children don't matter since gtk_container_destroy() has already removed them, but the internal child removals:

  if (priv->arrow_button)
    gtk_widget_unparent (priv->arrow_button);

and:

  if (priv->menu)
    {
      g_signal_handlers_disconnect_by_func (priv->menu,
                                            menu_deactivated, toolbar);
      gtk_widget_destroy (GTK_WIDGET (priv->menu));
    }

cause problems because finalize because they will cause ::parent-set to be emitted on the children, with the toolbar as an old_parent argument. And once you are in finalize with a refcount of zero, you can't use an object as a signal parameter.

*Mostly* this ends up not mattering since we short-circuit the ::parent-set if there are no signal handlers on the child, but if you have an overridden class closure or emission hook, you get a warning:

GLib-GObject-CRITICAL **: g_object_ref: assertion `object->ref_count > 0' failed

Will attach a program that demonstrates. The fix is to move the widget manipulation that's in gtk_toolbar_finalize() to a gtk_toolbar_dispose() (will need a little added safety against being run multiple times, such as nulling out priv->arrow-button.)