setTimeout 报错 undefined, 求解答

onPageScroll(e) {           if(this.Sid){               clearTimeout(this.Sid);           };           let st = flag => {               this.setData({                  isShowBackTop: flag               })           }           this.Sid = setTimeout(() => {               let top = this.data.isShowBackTop;               if(e && e.scrollTop > 400){                   !top && st(true);               }else{                   top && st(false);               }           },500);       },

错误样例: undefined;at page/hotel/pages/inland/promotions/bargain/index onPageScroll function;at setTimeout callback function

纠结了很久 就是找不到错误原因那！！！！  求官方解答。