export 'default' (imported as Vue ) was not found in 'vue'

I am new with this vue and I got this error when I try to run (npm run serve):

***WARNING Compiled with 4 warnings
warning in ./src/main.js
"export 'default' (imported as 'Vue') was not found in 'vue'
warning in ./src/main.js
"export 'default' (imported as 'Vue') was not found in 'vue'
warning in ./src/main.js
"export 'default' (imported as 'Vue') was not found in 'vue'
warning in ./src/router/index.js
"export 'default' (imported as 'Vue') was not found in 'vue'

App running at:

• Local: http://localhost:8080/
• Network: http://10.2.220.30:8080/***

index.js

import Vue from "vue";
import VueRouter from "vue-router";
import Home from "../views/Home.vue";
Vue.use(VueRouter);
const routes = [{
        path: "/home",
        name: "home",
        component: Home,
        meta: {
            requiresAuth: true
        }
    },
    {
        path: "/",
        name: "login",
        component: () =>
            import ("../views/login.vue")
    },
    {
        path: "/register",
        name: "register",
        component: () =>
            import ("../views/register.vue")
    }
];
const router = new VueRouter({
    mode: "history",
    base: process.env.BASE_URL,
    routes
});
router.beforeEach((to, from, next) => {
    if (to.matched.some(record => record.meta.requiresAuth)) {
        if (localStorage.getItem("jwt") == null) {
            next({
                path: "/"
            });
        } else {
            next();
        }
    } else {
        next();
    }
});
export default router;

main.js

import Vue from "vue";
import App from "./App.vue";
import router from "./router";
import axios from "axios";
import "bootstrap/dist/css/bootstrap.css";

const base = axios.create({
    baseURL: "http://localhost:4000"
});

Vue.prototype.$http = base;
Vue.config.productionTip = false;
new Vue({
    router,
    render: h => h(App)
}).$mount("#app");