title: Firebase missing initial state on auth request
body: <p>I am trying to use Apple sign-in with my Flutter/Firebase web app.  It works fine with a popup browser tab, so I know the general configuration is correct.  However, when the redirect flow is used instead of the popup, an error results.  When the Apple sign-in is complete, it makes a POST to Firebase's <code>https://YOUR_FIREBASE_PROJECT_ID.firebaseapp.com/__/auth/handler</code> URL which includes a <code>code</code> and <code>id_token</code>.  However, the Firebase Auth handler page gives the following error:</p>
  <p><code>Unable to process request due to missing initial state. This may happen if browser sessionStorage is inaccessible or accidentally cleared.</code></p>
  <p>Is there some configuration that I am missing to save the state in session storage before redirecting to the sign in page?  Or am I missing something else?</p>