app.js

import ReactRouterRelay from 'react-router-relay';

// ...

React.render(
  <Router
    history={new BrowserHistory()}
    createElement={ReactRouterRelay.createElement}>
    <Route
      name="home" // added a name to the route
      path="/"
      component={App}
      queries={AppQueries} // and the query
    />
    <Route
      name="widget"
      path="/widget/:id"
      component={Widget}
      queries={WidgetQueries}
    />
  </Router>,
  document.getElementById('root')
);

Probably best to change https://gist.github.com/cpojer/7b274ea7e255481e1d2d#file-app-js-L7 to history={history} given further API changes to react-router.