Gatsby error page which prevents a flash of 404

404.js

import React from "react";
import SEO from "../components/seo";

const browser = typeof window !== "undefined" && window;

const NotFoundPage = () => {
  return (
    browser && (
      <div>
        <SEO title="404 Page not found" />
        <h1>404 Error content...</h1>
      </div>
    )
  );
};

export default NotFoundPage;