vue 动态生成 el-checkbox-group v-model 绑定值的问题

Vue 不允许在已经创建的实例上动态添加新的根级响应式属性 (root-level reactive property)。然而它可以使用 Vue.set(object, key, value) 方法将响应属性添加到嵌套的对象上

<div v-if="item.questionType==1">
        <div v-for="(xx,yy) in item.children" :key="yy">
          <div v-html="(yy+1)+'、'+xx.text"></div>
          <div>
            <el-checkbox-group v-model="choiceMultiQuestion[yy]">
              <li v-for="(mm,nn) in xx.content.items" :key="nn">
                <el-checkbox :label="mm.prefix">
                  {{mm.prefix}}、
                  <span v-html="mm.content"></span>
                </el-checkbox>
              </li>
            </el-checkbox-group>
          </div>
        </div>
      </div>

for (let m in this.examination.children) {
  if (this.examination.children[m].questionType == 1) {
    for (let i in this.examination.children[m].children) {
       this.$set(this.choiceMultiQuestion, i, []);
     }
  }
}