Uncaught Error: Minified React error #200

My code like this, getting error Uncaught Error: Minified React error #200

how to fix this error anyone help me

thank you in advance
app.js

import React, { Component } from 'react';
import ReactDOM from 'react-dom';

class Mainapp extends Component {
render(){
return(

<h2>Main Page</h2>

    )
}

}

ReactDOM.render(,document.getElementById("main-content"));

index.html