title: "react-native run-android" gives an error on my mac
body: <p>Im  getting error on react native when run on my mac react-native run-android</p>
  
  <ul>
  <li><p>What went wrong:
  A problem occurred configuring project ':app'.</p>
  
  <blockquote>
    <p>java.lang.NullPointerException (no error message)</p>
  </blockquote></li>
  <li><p>Try:
  Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.</p></li>
  </ul>
  
  <p>BUILD FAILED</p>
  
  <p>Total time: 2 mins 15.025 secs
  Could not install the app on the device, read the error above for details.
  Make sure you have an Android emulator running or a device connected and have
  set up your Android development environment:
  <a href="https://facebook.github.io/react-native/docs/android-setup.html" rel="nofollow noreferrer">https://facebook.github.io/react-native/docs/android-setup.html</a></p>