message: Fix race when signup is completed elsewhere
  as commented
diff: @@ -350,13 +350,21 @@ module.exports = React.createClass({
  });
  break;
  case 'start_upgrade_registration':
  - // stash our guest creds so we can backout if needed
  + // also stash our credentials, then if we restore the session,
  + // we can just do it the same way whether we started upgrade
  + // registration or explicitly logged out
  this.guestCreds = MatrixClientPeg.getCredentials();
  this.setStateForNewScreen({
  screen: "register",
  upgradeUsername: MatrixClientPeg.get().getUserIdLocalpart(),
  guestAccessToken: MatrixClientPeg.get().getAccessToken(),
  });
  +
  + // stop the client: if we are syncing whilst the registration
  + // is completed in another browser, we'll be 401ed for using
  + // a guest access token for a non-guest account.
  + Lifecycle.stopMatrixClient();
  +
  this.notifyNewScreen('register');
  break;
  case 'start_password_recovery':
  