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

const Topic = ({match}) => console.log(match) || (
  <div>
    Topic
  </div>
);