错误＆＃34;元素类型无效＆＃34; reactjs项目

Question

一切都运转得很好，突然之间，我得到了令人难以置信的错误，回过头去做事，当事情正在发生时，我仍然得到这个错误，我不知道为什么< / p>

元素类型无效：期望一个字符串（对于内置组件）或一个类/函数（对于复合组件）但得到：undefined。您可能忘记从其定义的文件中导出组件。

检查p。

的渲染方法

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Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: undefined. You likely forgot to export your component from the file it's defined in.

Check the render method of `p`.
▶ 25 stack frames were collapsed.
./src/index.js
C:/Users/Jseto/Desktop/jsetome/src/index.js:27
  24 | 
  25 | const app = document.getElementById('root');
  26 | 
> 27 | ReactDOM.render(
  28 | 	<HashRouter>
  29 | 		<Switch>
  30 | 			<Route exact path="/" component={App}/>

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