[vue-router] TypeError: Cannot read property 'app' of undefined

1. When I return from the sub-routing page to the first-level routing page, there is an error
2. The browser console reported an error as follows

Uncaught (in promise) Error: [vue-router] TypeError: Cannot read property "app" of undefined

3. My code address
https://github.com/libp/littl.
4. Click on the route to achieve normal adjustment, and the page display is also normal, that is, the console reports an error
5.app.vue

<template>
  <div id="app">
    <router-view></router-view>
  </div>
</template>

<script>
export default {
  name: "App"
}
</script>

<style lang="scss" rel="stylesheet/scss">
  @import "./style/common.scss";
  .main {
    background: -sharpededed;;
  }
</style>

6. Report an error when clicking on the list of articles or recommendations

<el-submenu index="2">
            <template slot="title"><i class="el-icon-document"></i></template>
            <el-menu-item index="articleList"></el-menu-item>
            <el-menu-item index="recommendList"></el-menu-item>
          </el-submenu>

7.route.js is as follows

import Vue from "vue"
import Router from "vue-router"
const Index = () => import("/page/index.vue")
const Home = () => import("/page/Home/home.vue")
const Random = () => import("/page/Home/random.vue")
const About = () => import("/page/Home/about.vue")
const Num = () => import("/page/Home/num.vue")

const Nav = () => import("/page/ProgrammerNav/Nav.vue")
const Admin = () => import("/page/Admin/index.vue")
const Adminhome = () => import("/page/Admin/home.vue")
const ArticleList = () => import("/page/Admin/articlelist.vue")
const RecommendList = () => import("/page/Admin/recommendList.vue")

Vue.use(Router)

const scrollBehavior = function (to, from, savedPosition) {
  if (savedPosition) {
    // savedPosition is only available for popstate navigations.
    return savedPosition
  } else {
    const position = {}

    // scroll to anchor by returning the selector
    if (to.hash) {
      position.selector = to.hash

      // specify offset of the element
      if (to.hash === ".title") {
        position.offset = { y: 100 }
      }

      if (document.querySelector(to.hash)) {
        return position
      }

      // if the returned position is falsy or an empty object,
      // will retain current scroll position.
      return false
    }

    return new Promise(resolve => {
      // check if any matched route config has meta that requires scrolling to top
      if (to.matched.some(m => m.meta.scrollToTop)) {
        // coords will be used if no selector is provided,
        // or if the selector didn"t match any element.
        position.x = 0
        position.y = 0
      }

      // wait for the out transition to complete (if necessary)
      this.app.$root.$once("triggerScroll", () => {
        // if the resolved position is falsy or an empty object,
        // will retain current scroll position.
        resolve(position)
      })
    })
  }
}

export default new Router({
  mode: "history",
  scrollBehavior,
  routes: [
    {
      path: "/",
      component: Index,
      name: "index",
      redirect: "/home",
      children: [
        {path: "home", component: Home},
        {path: "random", component: Random, meta: { scrollToTop: true }},
        {path: "about", component: About, meta: { scrollToTop: true }},
        {path: "num/:id", name: "num", component: Num, meta: { scrollToTop: true }}
      ]
    },
    {
      path: "/nav",
      component: Nav
    },
    {
      path: "/dasheng",
      component: Admin,
      children: [
        {path: "/", component: Adminhome},
        {path: "/articleList", component: ArticleList, meta: ["", ""]},
        {path: "/recommendList", component: RecommendList, meta: ["", ""]}
      ]
    }
    // {
    //   path: "*",
    //   redirect: "/home"
    // }
  ]
})