Code does not check for session = null when obtaining webId

How to reproduce

1. Logged out and cleared localStorage
2. Attempted to login to https://melvin.solid.live/public/RWWCrew/
3. Click on custom domain, https://melvincarvalho.com/#me

Observed behaviour

Uncaught (in promise) TypeError: Cannot read property 'webId' of null
    at signin.js:732
    at <anonymous>

in

  signInPopUpButton.addEventListener('click', function () {
    var offline = offlineTestID();
    if (offline) return setUserCallback(offline.uri);
    return solidAuthClient.popupLogin({ popupUri: $SOLID_GLOBAL_config.popupUri }).then(function (session) {
      var webIdURI = session.webId;

Expected Behaviour

What I would expect is to redirect me to my oidc provider, let me log in, and then give me a bearer token

Possible fix

https://github.com/solid/solid-ui/blob/master/src/signin.js#L739 should use webIdFromSession

Start at src/signin.js around the signInPopUpButton handler near line 739, then compare its session handling with webIdFromSession near line 783. Reproduce the logged-out custom-domain login flow and verify that a null session no longer causes the reported webId TypeError and that the expected provider login and bearer-token flow can proceed.