react-children期望收到单个反应元素子错误

Question

我正在尝试使用React，Redux创建一个调查应用。尝试在localhost:3000上运行该程序时，我收到此React.Children.only expected to receive a single React element child错误。我尝试过使用程序员在别处提到的各种解决方案，但没有任何效果。代码如下。我必须承认，就编程而言，我是一个绝对的tyro

App.js：

import React from 'react';
import {BrowserRouter,Route} from 'react-router-dom';
import Header from './Header';


const Dashboard =() => <h2>Dashboard</h2>;
const SurveyNew =() => <h2>SurveyNew</h2>;
const Landing =() => <h2>Landing</h2>;

const App = () => {
 return (
 <div>
  <BrowserRouter>
    <div>
      <Header />
      <Route exact path= "/" component = {Landing} />
      <Route exact path = "/surveys" component= {Dashboard} />
      <Route path = "/surveys/new" component={SurveyNew} />
    </div> 
  </BrowserRouter>
 </div>
 );

};

export default App; 

header.js文件的代码如下：

import React, {Component} from 'react';
class Header extends Component {

 render () {
 return (
  <div>
   <nav>
    <div className = "nav-wrapper">
      <a className = "left brand-logo">
      Emaily
      </a>
       <ul className="right">
        <li>
         <a> Login with Google </a>
        </li>
       </ul>
    </div>
   </nav>
  </div>
 );
}

}

export default Header;