...
const Topics = ({match}) => (
  <div>
    Topics
    <ul>
      <li><Link to={`${match.url}/render-with-react`}>Render with React</Link></li>
      <li><Link to={`${match.url}/components`}>Components</Link></li>
      <li><Link to={`${match.url}/props-vs-state`}>Props vs State</Link></li>
    </ul>
    <Route path="/topics/:topicId" component={Topic} />
  </div>
);