title: reactjs - valid React element (or null) must be returned
body: <p>I have the following simple code :</p>
  
  <pre><code>var data = [{ email: "bob@gmail.com" },{ email: "toto@gmail.com" }];
  var User = React.createClass({
  render: function ()
  {
      return
      (
          &lt;li&gt;
              {this.props.email}
          &lt;/li&gt;
      );
  }});
  
  var UserList = React.createClass({        
  render: function ()
  {            
      var userNodes = this.props.data.map(function (user)
      {
          console.log(user.email);
          return
          (
              &lt;User email={user.email} &gt;&lt;/User&gt;
          );
      });            
      return
      (
          &lt;ul&gt;{userNodes}&lt;/ul&gt;
      );
  }});
  
  
  ReactDOM.render(&lt;UserList data={data} /&gt;, document.getElementById('root'));
  </code></pre>
  
  <p>and I got this error:</p>
  
  <blockquote>
    <p>"A valid React element (or null) must be returned. You may have returned undefined, an array or some other invalid object." </p>
  </blockquote>