в чем ошибка TypeError: Cannot read property 'count' of null

import React, { Component } from 'react';
import ReactDOM from 'react-dom';
import * as serviceWorker from './serviceWorker';
import { observable } from "mobx"
import {observer} from "mobx-react";


 @observer class App extends Component {
     @observable     state={
           count:  0
   }

        render() {
    return (
      <div className="App">

        <h1>{this.state.count}</h1>
<button onClick={()=>this.setState({count:this.state.count +1})}>fefefef</button>

      </div>
    );
  }
}

ReactDOM.render(<App />, document.getElementById('root'));

serviceWorker.unregister();