message: fix not showing 'Checking...'
  The initial state of `pass` is `undefined`, not `null`.
diff: @@ -26,7 +26,7 @@ export default () => {
  return (
  <div>
  - {pass === null && 'Checking...'}
  + {pass === undefined && 'Checking...'}
  {pass === true && 'Check passed.'}
  </div>
  );
  