fix[DragSelect]: fixed querySelectorAll bug

Author: PanJiaChen

src/components/DragSelect/index.vue

@@ -1,5 +1,5 @@
 <template>
-  <el-select v-model="selectVal" v-bind="$attrs" class="drag-select" multiple>
+  <el-select ref="dragSelect" v-model="selectVal" v-bind="$attrs" class="drag-select" multiple>
     <slot/>
   </el-select>
 </template>
@@ -30,7 +30,7 @@ export default {
   },
   methods: {
     setSort() {
-      const el = document.querySelectorAll('.el-select__tags > span')[0]
+      const el = this.$refs.dragSelect.$el.querySelectorAll('.el-select__tags > span')[0]
       this.sortable = Sortable.create(el, {
         ghostClass: 'sortable-ghost', // Class name for the drop placeholder,
         setData: function(dataTransfer) {