title: The prop `children` is marked as required in `Button`, but its value is `undefined`
body: <p>I am trying to convert an older Material UI implementation. The Docs are bit laggy.</p>
  
  <p>I get this error. The docs say</p>
  
  <blockquote>
    <p>Name          Type    Default Description </p>
    
    <p>children *    node        The content of the
    button.</p>
  </blockquote>
  
  <p>What does this mean and what do I have to put in my code?</p>
  
  <pre><code>&lt;Button
    variant="raised"
    color="primary"
    className={classes.Button}
    label={this.state.buttonLabel}
    onClick={this.handleClick}
  &gt;
  &lt;/Button&gt;
  </code></pre>