a11y: return -1 if parent is NULL
Fixes:
(gnome-text-editor:1929283): Gtk-CRITICAL **: 11:55:22.093: gtk_widget_get_first_child: assertion 'GTK_IS_WIDGET (widget)' failed
Fixes:
(gnome-text-editor:1929283): Gtk-CRITICAL **: 11:55:22.093: gtk_widget_get_first_child: assertion 'GTK_IS_WIDGET (widget)' failed