Comment 13 for bug 529052

Both of those error messages occur internally to libappindicator because the code assumes GtkStatusIcon is a subclass of GtkWidget, which is not the case. This is the bit which fails (it causes those messages AND the extra space to appear in the notification area):

gtk_widget_hide(GTK_WIDGET(status_icon));

This should be:

gtk_status_icon_set_visible (status_icon, FALSE);