[LayaAir 2.0]Button销毁时，未对_clickHandler进行回收

 destroy(destroyChild = true) {

            super.destroy(destroyChild);

            this._bitmap && this._bitmap.destroy();

            this._text && this._text.destroy(destroyChild);

            this._bitmap = null;

            this._text = null;

            this._clickHandler = null;

            this._labelColors = this._sources = this._strokeColors = null;

}