title: TypeError: Cannot read property 'props' of null in react
body: <p>error:
  Uncaught TypeError: Cannot read property 'props' of null
      at babaClick (:75:17)
  If i type it in ES5 it won't be wrong but if i type it in es6 just like the following code it's wrong .I just don't know why...</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html lang="en"&gt;
  &lt;head&gt;
  &lt;meta charset="UTF-8"&gt;
  &lt;title&gt;HelloMessage&lt;/title&gt;
  &lt;script src="lib/react.js"&gt;&lt;/script&gt;
  &lt;script src="lib/react-dom.js"&gt;&lt;/script&gt;
  &lt;script src="lib/babel.min.js"&gt;&lt;/script&gt;
  &lt;script type="text/babel"&gt;
      class HelloMesssage extends  React.Component{
          constructor(props){
              super(props)
              this.state={
                  zan:0
              }
  
          }
          addZan(){
              this.setState(
                      {
                          zan:this.state.zan+1
                      }
              )
          }
          render(){
              return(
                      &lt;div&gt;
                          &lt;p&gt;姓名：{this.props.name}&lt;/p&gt;
                          &lt;p&gt;赞：&lt;Zan zan={this.state.zan}/&gt;&lt;/p&gt;
                          &lt;p&gt;&lt;Btn babaZan={this.addZan.bind(this)}/&gt;&lt;/p&gt;
                      &lt;/div&gt;
              )
          }
      }
      class Btn extends  React.Component{
          babaClick(){
              this.props.babaZan();
          }
          render(){
              return(
                      &lt;button onClick={this.babaClick}&gt;赞&lt;/button&gt;
              )
          }
  
      }
      class Zan extends  React.Component{
          render(){
              return(
                      &lt;span&gt;{this.props.zan}&lt;/span&gt;
              )
          }
      }
      ReactDOM.render(
              &lt;HelloMesssage name="xxxindy"/&gt;,
              document.getElementById('container')
      );
  &lt;/script&gt;
  &lt;div id="container"&gt;&lt;/div&gt;
  </code></pre>
  
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