(WebKitPluginProcess:31915): Gtk-CRITICAL **: gtk_widget_destroy: assertion 'GTK_IS_WIDGET (widget)' failed
This is because our plugin widget that is a GtkPlug (derived from GtkWindow) can receive the delete-event signal before NetscapePlugin::platformDestroy is called. The delete-event signal, by default, destroys the window when the signal is not handled. So after the delete-event the GtkPlug is destroyed, but our pointer hasn't been reset. We can handle the delete-event using gtk_widget_hide_on_delete as callback, so that the plugin widget is hidden instead of destroyed.
Created attachment 265280 [details]
Committed r192311: <http://trac.webkit.org/changeset/192311>