title: The prop `children` is marked as required in `AppBar`, but its value is `undefined`
body: <p>I want to use native Material-ui component but have this error in browser :</p>
  
  <p>index.js:2178 Warning: Failed prop type: The prop <code>children</code> is marked as required in <code>AppBar</code>, but its value is <code>undefined</code>.</p>
  
  <p>My component is:</p>
  
  <pre><code>import React from 'react';
  import AppBar from 'material-ui/AppBar';
  
  /**
   * A simple example of `AppBar` with an icon on the right.
   * By default, the left icon is a navigation-menu.
   */
  const Header = () =&gt; (
    &lt;AppBar
      title="Title"
      iconClassNameRight="muidocs-icon-navigation-expand-more"
    /&gt;
  );
  
  export default Header;
  </code></pre>
  
  <p>Help me to understand why?</p>