title: Firebase email link authentication leads to a page that says "Error encountered" - "The selected page mode is invalid"
body: <p>I am on localhost with Firebase auth (firebase v7.23.0). The email is sent okay. I click the sign in link from my email. I get this error page on my &lt;PROJECT&gt;.firebaseapp.com</p>
  <p><a href="https://i.stack.imgur.com/FU0vu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/FU0vu.png" alt="!" /></a></p>
  <p>My code is mostly below, but this all works. It's the Firebase page is giving an error. I am not sure what else I need to set up. localhost is on the allowed domains. The error is on the Firebase side so I cannot do much to debug or fix it.</p>
  <pre><code>const actionCodeSettings = {
    // URL you want to redirect back to. The domain (www.example.com) for this
    // URL must be whitelisted in the Firebase Console.
    // url: `${window.location.protocol}//${window.location.host}/`,
    url: http://localhost:7000/#postLogin,
    // This must be true.
    handleCodeInApp: true
  };
  
  firebase.auth().sendSignInLinkToEmail(this.email, actionCodeSettings)
          .then(() =&gt; {
            // The link was successfully sent. Inform the user.
            // Save the email locally so you don't need to ask the user for it again.
            window.localStorage.setItem('email', this.email);
          })
          .catch(error =&gt; {
            // Some error occurred, you can inspect the code: error.code
          });
      }
  </code></pre>