title: input's event.target is null within this.setState [React.js]
body: <p>In my react component I have a file input: </p>
  
  <pre><code>&lt;input type="file" onChange={this.onFileChange.bind(this)} /&gt;` 
  </code></pre>
  
  <p>and my <code>onFileChange</code> is:</p>
  
  <pre><code>onFileChange(e) {
    let file = e.target.files[0];
    this.setState(() =&gt; ({ file: e.target.files[0] })); //doesnt work
    // this.setState(() =&gt; ({ file })); //works
    // this.setState({ file: e.target.files[0] }); //works
  }
  </code></pre>
  
  <p>This first way of setting the states fails with an error: </p>
  
  <pre><code>Cannot read property 'files' of null
  </code></pre>
  
  <p>React also gives the following warning: </p>
  
  <pre><code>This synthetic event is reused for performance reasons. If you're 
  seeing this, you're accessing the property 'target' on a 
  released/nullified synthetic event
  </code></pre>
  
  <p>But the last two ways of setting the state give no errors or warnings. Why is this happening? </p>