this.scroll = new BScroll(this.$refs.xxx)报错解决办法

近期做一个城市列表的小demo，用到了第三方的插件better-scroll（如何插入请参考：uni-app引用npm第三方库）；
费时1天，解决报错问题：
错误问题:
Cannot read property '0' of undefined;at pages/city/city page lifeCycleMethod onReady function
TypeError: Cannot read property '0' of undefined
at new K (http://127.0.0.1:20155/appservice/pages/city/city.js:9:12181)
at a.mounted (http://127.0.0.1:20155/appservice/pages/city/city.js:9:56084)
at Gt (http://127.0.0.1:20155/appservice/common/vendor.js:2:16693)
错误代码：

this.scroll = new BScroll(this.$refs.wrapper)

错误原因：
this.$refs.wrapper 为 {}

解决办法：

            if (JSON.stringify(this.$refs.wrapper)=='{}') {  
                return;  
            }  
            this.scroll = new BScroll(this.$refs.wrapper)