React 0.14.8
We see this in browser tests but never manually triggered, so bug is seemingly timing sensitive.

```
Uncaught TypeError: Cannot read property 'classList' of null CSSCore.js:38
CSSCore.addClass @ CSSCore.js:38
React.createClass.flushClassNameQueue @ ReactCSSTransitionGroupChild.js:116
```

`component.isMounted()` is `true`, but in these cases `ReactDOM.findDOMNode(this)` then returns null here
https://github.com/facebook/react/blob/master/src/addons/transitions/ReactCSSTransitionGroupChild.js#L116

this.transitionTimeouts.push(timeout);
    } else {
      
      ReactTransitionEvents.addEndEventListener(node, endListener);
    }
  },
  queueClass: function(className) {
    this.classNameQueue.push(className);
    if (!this.timeout) {
      this.timeout = setTimeout(this.flushClassNameQueue, TICK);
    }
  },
  flushClassNameQueue: function() {
    if (this.isMounted()) {
      this.classNameQueue.forEach(
        CSSCore.addClass.bind(CSSCore, ReactDOM.findDOMNode(this))
      );
    }
    this.classNameQueue.length = 0;
    this.timeout = null;
  },
  componentWillMount: function() {
    this.classNameQueue = [];
    this.transitionTimeouts = [];
  },
  componentWillUnmount: function() {
    if (this.timeout) {
      clearTimeout(this.timeout);
    }
    this.transitionTimeouts.forEach(function(timeout) {