element ui日期选择器清除后报错了

<div class="block">
    <span class="demonstration">开始日期</span>
    <el-date-picker
      v-model="value1"
      type="date"
      placeholder="选择日期"
      @change="startPicker1"
    >
    </el-date-picker>
  </div>
  <div class="block">
    <span class="demonstration">结束日期</span>
    <el-date-picker
      v-model="value2"
      align="right"
      type="date"
      placeholder="选择日期"
      @change="startPicker2"
    >
    </el-date-picker>
  </div>

  //点击清除按钮时，value值会被设置为null。
  //方法是在下次调用之前，为value重新赋值
  methods:{
    startPicker1(date){
      if(!date){this.value1=''}
    },
    startPicker1(date){
      if(!date){this.value2=''}
    }

  }

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