title: Invariant Violation: "main" has not been registered
body: <p>New to React Native:
  I started a brand new project with Expo init and then I followed  the instructions mentioned inhttps://reactnavigation.org/docs/hello-react-navigation
  I run the project with expo start and I get this error.
  Invariant Violation: &quot;main&quot; has not been registered. This can happen if:</p>
  <ul>
  <li>Metro (the local dev server) is run from the wrong folder. Check if Metro is running, stop it and restart it in the current project.</li>
  <li>A module failed to load due to an error and <code>AppRegistry.registerComponent</code> wasn't called.</li>
  </ul>
  <p>invariant
  browser.js:38:14
  runApplication
  AppRegistry.js:193:13
  __callFunction
  MessageQueue.js:425:19
  __guard$argument_0
  MessageQueue.js:112:6
  __guard
  MessageQueue.js:373:10
  callFunctionReturnFlushedQueue
  MessageQueue.js:111:4
  callFunctionReturnFlushedQueue
  [native code]:0
  Any suggestions?</p>