如何在组件vue上解决“Uncaught TypeError：无法读取属性'添加'的null”？

Question

我的vue组件是这样的：

<template>
    <div class="row">
        <div class="col-xs-6 col-sm-6 col-md-3" v-for="item in items">
            <!-- this is images  slider -->
        </div>
    </div>
</template>
<script>
    export default {
        ...
        mounted() {
            this.getTopLeague([{league_id: this.leagueId}]) // this is ajax if load first
            setTimeout( function(){ 
                $('.slick').slick({slidesToShow: 4, infinite: false});
            }  , 10000 ); // I set 10 seconds for the finished ajax process just started the slider
        },
        ...
        methods: {
            changeLeague(leagueId) {
                this.getTopLeague([{league_id: leagueId}]) // this is ajax if a link clicked    
                setTimeout( function(){ 
                    $('.slick').slick({slidesToShow: 4, infinite: false});
                }  , 10000 );  //I set 10 seconds for the finished ajax process just started the slider
            }
        }
    }
</script>

如果执行了代码，则会出现如下错误：

  

未捕获的TypeError：无法读取null的属性“add”       在jQuery.fn.init.a.fn.slick

10秒或ajax过程结束后发生错误

我将滑块设置为在10秒后运行，因为如果ajax进程尚未完成，则会出现错误

我该如何解决这个问题？