title: react-native-webview this.webView.postMessage not working
body: <p>The previous version does not work as it is new.</p>
  
  <p>"react-native-webview": "^9.0.2"
  "react-native": "0.62.0",</p>
  
  <pre><code>2. webview options
  javaScriptEnabled={true}
  mediaPlaybackRequiresUserAction={false}
  startInLoadingState={true}
  allowUniversalAccessFromFileURLs={true}
  onMessage={this.respondToOnMessage}
  
  3. react-native code
  this.webView.postMessage( "Post message from react native" );
  
  4. web code
  
  function receiveMessage(e) {
      callbackFunction(JSON.parse(e.data));
  }
  function RNPostMessage(data) {
      window.ReactNativeWebView.postMessage(data);
  }
  window.addEventListener('message', receiveMessage, true);
  
  5. react-native error msg
  D:\dev\projects\mobile-app\react-native\yc_attend\node_modules\react-devtools-core\dist\backend.js:32 Possible Unhandled Promise Rejection (id: 0):
  TypeError: Cannot read property 'postMessage' of undefined
  TypeError: Cannot read property 'postMessage' of undefined
  </code></pre>
  
  <p>I want to know the solution to this...</p>