[Bug Report] DatetimeView组件触发render方法时报错

[luhnn120]

VUX version

2.7.7

OS/Browsers version

win/chrome

Vue version

2.5.13

Code

<template>
  <div>
    <datetime-view ref="datetime" v-model="time" :format='format'></datetime-view>
    <button style="width:200px;height:100px;font-size:20px" @click="clickButton">test</button>
  </div>
</template>
<script>
  import {  DatetimeView } from 'vux'
  export default {
    data() {
      return {
          time: '2017-10-17',
          format: 'YYYY-MM-DD'
      }
    },
    methods: {
        clickButton(){
            this.time = '2019-10-23 10'
            this.format = 'YYYY-MM-DD HH'
            this.$refs.datetime.render()
        }
    },
    components: {
      DatetimeView
    }
  }

</script>
<style>
</style>

Steps to reproduce

1.页面开始时DatetimeView组件定位到设置的时间2017-10-17
2.点击按钮test
3.控制台报错且DatetimeView组件没有定位到设置时间2019-10-23 10
报错信息：
Uncaught TypeError: Cannot read property 'style' of null
at eval (datetimepicker.js?cd5e:185)

What is Expected?

点击按钮test后控制台不报错，且DatetimeView组件能够定位到设置的时间处

What is actually happening?

DatetimeView组件没有能够定位到设置的时间处，且控制台报错。
报错信息：
Uncaught TypeError: Cannot read property 'style' of null
at eval (datetimepicker.js?cd5e:185)

[unclay]

已修复，下版本更新
format是响应属性，可以先设置，在设置value

this.format = 'YYYY-MM-DD HH'
this.$nextTick(() => {
  this.value1 = '2019-10-23 10'
  this.$refs.datetime.render()
})