Guard onLoggedIn since registration uses it too and that isn't done yet

src/controllers/pages/MatrixChat.js

@@ -294,11 +294,13 @@ module.exports = {
     },
 
     onLoggedIn: function(credentials) {
+        if (credentials) { // registration doesn't do this yet
             console.log("onLoggedIn => %s", credentials.userId);
             MatrixClientPeg.replaceUsingAccessToken(
                 credentials.homeserverUrl, credentials.identityServerUrl,
                 credentials.userId, credentials.accessToken
             );
+        }
         this.setState({
             screen: undefined,
             logged_in: true