在react-router v4中看到嵌套路由中的空白页

import React from 'react';
import { Switch, Route } from 'react-router-dom';

const App = () => (
  <div>
    <h1>My App</h1>
    <Switch>
      <Route exact path="/" component={Home} />
      <Route path="/about" component={About} />
      <Route path="/contact" component={Contact} />
      <Route component={NotFound} /> {/* 空白页路由 */}
    </Switch>
  </div>
);

const Home = () => <h2>Home</h2>;
const About = () => <h2>About</h2>;
const Contact = () => <h2>Contact</h2>;
const NotFound = () => <h2>Page Not Found</h2>;

export default App;