TypeError: ref is undefined after ZK-3943

zk.afterLoad('zul.wgt', function () {
    var xNotification = {};
    zk.override(zul.wgt.Notification.prototype, xNotification, {
        reposition: function () {
            this.$supers('reposition', arguments);
            if (this._ref) this._fixarrow(this._ref);
        }
    });
});