cytoscape: TypeError Cannot read properties of null (reading 'className')

Question

I want to show the graph in a dialog with cytoscape, I simplify the code as following:

<template>
  <div>
    <el-dialog
        title="提示"
        :visible.sync="dialogVisible"
        width="80%"
        :fullscreen="true">
      <div id="cy" class="testclass" style="width: 100%;height: 100%"/>
    </el-dialog>
  </div>
</template>

<script>
// import bus from "@/assets/bus";
import {address} from "@/util/config";
import cytoscape from "cytoscape";

export default {
  name: "GraphDialog",
  data() {
    return {
      dialogVisible: false,
      cy: {}
    }
  },

  mounted() {
    // bus.$on('showGraphDialog', (data) => {
    //   console.log(data);
    let data = {"code": "01", "name": "生态环境质量"}
    this.$axios({
      method: 'get',
      params: {
        code: data.code,
        name: data.name
      },
      url: address + '/node/detail',
    }).then(res => {
      console.log(res)
      // this.elements = res.data.data
      console.log(document.getElementById('cy'))
      this.cy = this.$cytoscape({
        container: document.getElementById('cy'),     // 定义需要渲染的容器
        layout: {name: 'cose', fit: true},  //画布自适应大小
        elements: res.data.data
      })
      this.dialogVisible = true;

    })
    // });
  }
}
</script>


<style scoped>
</style>

But the error

 cytoscape.cjs.js?44e1:26943 Uncaught (in promise) TypeError: Cannot read properties of null (reading 'className')
    at Renderer.BRp$f.init (cytoscape.cjs.js?44e1:26943:1)
    at Renderer.BaseRenderer (cytoscape.cjs.js?44e1:26923:1)
    at new Renderer (cytoscape.cjs.js?44e1:31698:1)
    at Core.initRenderer (cytoscape.cjs.js?44e1:14453:1)
    at new Core (cytoscape.cjs.js?44e1:18641:1)
    at VueComponent.cytoscape [as $cytoscape] (cytoscape.cjs.js?44e1:31885:1)
    at eval (GraphDialog.vue?c794:136:1)

occured and I don't know why. And I alse tried the getDocumentById function doesn't work either in mounted or in the ajax of mounted. any help will be grateful.......