title: If proptype of element what is the default?
body: <p>if I try to set a proptype as <code>PropTypes.element</code>, not required, what is the proper default?</p>
  
  <pre><code>static propTypes = {
      expandable: PropTypes.bool,
      popover: PropTypes.element,
    }
  
    static defaultProps = {
      expandable: false,
      popover: () =&gt; {},
    }
  </code></pre>
  
  <p>Thanks</p>