Use Ref focus then it can not suggest

[greeenSY]

I'm submitting a ...

• bug report
• feature request
• support request

What is the current behavior?

I emit a focus event, then it errors when suggest.

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

<template>
   <vue-simple-suggest
              v-if="emailInputVisible"
              ref="emailInput"
              type="text"
              size="small"
              v-model="emailInputValue"
              :list="suggestedEmailList"
              :style="{ width: '240px', height: '25px', marginRight: '8px', display: 'inline-block' }"
              autocomplete="off"
              :filter-by-query="true"
          >
          </vue-simple-suggest>
          <a-tag v-else style="background: #fff; borderStyle: dashed;" @click="showEmailInput">
            <a-icon type="plus"/>
          </a-tag>
</template>

<script>
export default {
  //...
  
  data() {
    emailInputValue: '',
    suggestedEmailList: []
  }
  methods: {
    showEmailInput() {
      this.emailInputVisible = true
      this.$nextTick(function () {
         this.$refs.emailInput.$el.querySelector('input').focus() // it encountered this error？
      })
    },
  }
  //...
}
</script>

What is the expected behavior?

How are you importing Vue-simple-suggest?

• ESNext (original code, single-file .vue component, css included) (import VueSimpleSuggest from 'vue-simple-suggest/lib')
• ES6 (import VueSimpleSuggest from 'vue-simple-suggest')
• ES7 and above (import VueSimpleSuggest from 'vue-simple-suggest/dist/es7')
• Bundled version (import VueSimpleSuggest from 'vue-simple-suggest')
• CommonJS (const VueSimpleSuggest = require('vue-simple-suggest'))
• UMD Component (<script type="text/javascript" src="https://unpkg.com/vue-simple-suggest"></script>)

What is the motivation / use case for changing the behavior?

Please tell us about your environment:

• Vue.js Version: 2.6.10
• Vue-simple-suggest version: 1.11.2
• Browser: Chrome
• Language: ES6

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, gitter, etc)