title: ReactNativeWebView is Not Recognised
body: <p>I’m using <code>react-native-webview</code> and attempting to post a message from JavaScript to RN.</p>
  
  <p>I’m using the <code>window.ReactNativeWebView.postMessage()</code> method but cannot get beyond some issue with <code>ReactNativeWebView</code> not being recognised, with the following error:</p>
  
  <pre><code>Cannot read property 'postMessage' of undefined
  </code></pre>
  
  <p>I’m using the following:</p>
  
  <ul>
  <li>"react": "16.8.3",</li>
  <li>"react-native": "0.59.8",</li>
  <li>"react-native-webview": "^5.12.0",</li>
  </ul>
  
  <p>This looks so straight forward so I don’t know where I’m going wrong.</p>