How to share Authentication token between app in Firebase 3.0?
<p>In older version of Firebase we can obtain authenticated token for sharing between our app, extensions.</p>

<p>But when upgrade to Firebase 3.0, that function doesn't work anymore.
I've use getTokenWithCompletion: on FIRUser, then in my App Extension I call signInWithCustomToken:completion: with token i just obtained. But Firebase return an error with messgage:</p>

<blockquote>
  <p>Error Domain=FIRAuthErrorDomain Code=17000 "The custom token format is
  incorrect. Please check the documentation." UserInfo=0x799b6010
  {error_name=ERROR_INVALID_CUSTOM_TOKEN, NSLocalizedDescription=The
  custom token format is incorrect. Please check the documentation.}</p>
</blockquote>

<p>How to get authenticate token from FIRUser to re-authenticate it in my App Extension?</p>