title: Uncaught TypeError: Cannot read property 'func' of undefined
body: <p>I am using propTypes func and this is giving me an error only when I run the app</p>
  
  <p>After I ran: <strong><em>> npm run watch</em></strong></p>
  
  <p>I have no errors and everything seems to be "perfect". But when I ran the app I get the following error: </p>
  
  <blockquote>
    <p><em>Uncaught TypeError: Cannot read property 'func' of undefined</em></p>
  </blockquote>
  
  <p>This is the piece of code that used, but still don't understand why I get this error.</p>
  
  <pre><code>PlanFinder.propTypes = {
      onClickBubbles: React.propTypes.func,
  }
  </code></pre>