title: i write a react demo Using react-router,why the ticket route is null? the react-router-dom version is 4.2.2
body: <p>i write a react demo 
  Using react-router,why the ticket route is null?  the react-router-dom version is 4.2.2. </p>
  
  <pre><code>&lt;Router history={ReactHistory}&gt;
                  &lt;div&gt;
                      &lt;Route exact path="/" component={counter}/&gt;
                      &lt;Route path="/Ticket" component={test}/&gt;
                  &lt;/div&gt;&lt;/Router&gt;
  </code></pre>
  
  <p>the ticket route is null React Developer Tools.</p>
  
  <p>Thank you!</p>