iview 下拉刷新loadTop报错解决

<div class="noData" v-if="lifeList.length==0">
<img src="../assets/images/noData.png" alt="">
<p>这里暂时还没有内容哦~亲</p>
</div>
<mt-loadmore :top-method="loadTop" ref="loadTop" v-else>
<div v-infinite-scroll="loadMore" infinite-scroll-disabled="loading" infinite-scroll-distance="10">
<LifeListItem :lists="lifeList" merchantType="user"></LifeListItem>
<div class="loading-text" v-show="{loadingTextBtn:true}">
<span v-text="loadingText"></span>
<mt-spinner v-if="(loadingComplete==false)" type="snake" :size="16"></mt-spinner>
</div>
</div>
<div class="refreshComplete" v-if="refreshComplete==true">帖子已更新</div>
</mt-loadmore>

Error in mounted hook: "TypeError: Cannot read property 'onTopLoaded' of undefined"

loadTop(){
        this.$store.dispatch('refreshLifeList', {
          city: this.city,
          country: this.country,
          category: this.category,
          page: 0,
          size: this.size
        });
        this.page++;
        if(this.lifeList.length>0){ //这个判断是新加的
          this.$refs.loadTop.onTopLoaded();
        }
        this.refreshComplete = true;
        setTimeout(() => {
          this.refreshComplete = false;
        }, 2000)
      },