title: React-Native Linking AddEventListener not working
body: <p>Hi I was trying to use React-Native's Linking library to listen to Linking changes and I followed the instructions on <a href="https://facebook.github.io/react-native/docs/linking.html" rel="noreferrer">https://facebook.github.io/react-native/docs/linking.html</a>. I can open external URL using openURL but Linking.addEventListener doesn't seem to work for me. 
  I copied the code snippet:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>componentDidMount() {
    Linking.addEventListener('url', this._handleOpenURL);
  },
  componentWillUnmount() {
    Linking.removeEventListener('url', this._handleOpenURL);
  },
  _handleOpenURL(event) {
    console.log(event.url);
  }</code></pre>
  </div>
  </div>
  </p>
  
  <p>it doesn't give me an error but the _handleOpenURL is not called when the app opens up a external URL.</p>
  
  <p>I wonder why is this case and what should I do to fix it?</p>