如何从express发送默认的nextjs 404未找到错误页面？

app.use(function(req, res, next) {
  res.status(404).send('404 Not Found');
});

import React from 'react';

const NotFoundPage = () => {
  return (
    <div>
      <h1>404 Not Found</h1>
      <p>Sorry, the page you are looking for does not exist.</p>
    </div>
  );
};

export default NotFoundPage;

import React from 'react';
import App from 'next/app';

class CustomApp extends App {
  static async getInitialProps({ Component, ctx }) {
    let pageProps = {};

    if (Component.getInitialProps) {
      pageProps = await Component.getInitialProps(ctx);
    }

    return { pageProps };
  }

  render() {
    const { Component, pageProps } = this.props;

    return <Component {...pageProps} />;
  }
}

export default CustomApp;

module.exports = {
  // ...
  async rewrites() {
    return [
      {
        source: '/404',
        destination: '/_error',
      },
    ];
  },
};