title: Material-UI: the value provided `undefined` is invalid
body: <p>I use <strong>Tabs</strong>, <strong>Tab</strong> and <strong>Appbar</strong> from  material ui 1.0.0-beta.35 to display some tabs with labels imported from an object with data.</p>
  
  <p>I get this error from material-ui:</p>
  
  <pre><code>Warning: Material-UI: the value provided `undefined` is invalid
  </code></pre>
  
  <p>I could reproduce it in this <strong>codesandbox</strong> (material ui 1.0.0-beta.38 but same error):</p>
  
  <p><a href="https://codesandbox.io/s/rl9w5wkovq" rel="noreferrer">code example</a></p>
  
  <p>Click "Console" to see the errors.
  I have followed the guidelines as far as I can see? :</p>
  
  <p><a href="https://material-ui-next.com/api/tabs/" rel="noreferrer">https://material-ui-next.com/api/tabs/</a></p>
  
  <p><a href="https://material-ui-next.com/api/tab/" rel="noreferrer">https://material-ui-next.com/api/tab/</a></p>
  
  <p><a href="https://material-ui-next.com/api/app-bar/" rel="noreferrer">https://material-ui-next.com/api/app-bar/</a></p>
  
  <p>On the real project this causes the appbar to not function when I click the tabs. It did work for some time, but then this error happened out of the blue, but I haven't been able to find what is causing the error.</p>