title: Unable to get property 'remove' of undefined or null reference
body: <p>According to my client-side error logs, my ReactJS app is sometimes throwing the following exception.</p>
  
  <pre><code>TypeError: Unable to get property 'remove' of undefined or null reference
     at M.willDeleteListener (eval code:17:25544)
     at v.deleteAllListeners (eval code:1:25843)
     at m.Mixin.unmountComponent (eval code:16:15442)
     at i.unmountComponent (eval code:15:5262)
     at unmountChildren (eval code:17:3368)
     at _.Mixin.unmountChildren (eval code:17:2153)
     at m.Mixin.unmountComponent (eval code:16:15442)
     at i.unmountComponent (eval code:15:5262)
     at _.unmountComponent (eval code:15:16416)
     at i.unmountComponent (eval code:15:5262)
  </code></pre>
  
  <p>I think the source of the error is in <a href="https://github.com/facebook/react/blob/35962a00084382b49d1f9e3bd36612925f360e5b/src/renderers/dom/client/eventPlugins/SimpleEventPlugin.js#L600">https://github.com/facebook/react/blob/35962a00084382b49d1f9e3bd36612925f360e5b/src/renderers/dom/client/eventPlugins/SimpleEventPlugin.js#L600</a></p>
  
  <p>I've been unable to reproduce (either locally or in production).</p>
  
  <p>What could be causing this problem to happen? I need some ideas of things to try that could reproduce the error.</p>