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

if (!node) {
      if (finishCallback) {
        finishCallback();
      }
      return;
    }
    var className = this.props.name[animationType] || this.props.name + '-' + animationType;
    var activeClassName = this.props.name[animationType + 'Active'] || className + '-active';
    var timeout = null;
    var endListener = function(e) {
      if (e && e.target !== node) {
        return;
      }
      clearTimeout(timeout);
      CSSCore.removeClass(node, className);
      CSSCore.removeClass(node, activeClassName);
      ReactTransitionEvents.removeEndEventListener(node, endListener);
      
      
      if (finishCallback) {
        finishCallback();
      }
    };
    CSSCore.addClass(node, className);
    
    this.queueClass(activeClassName);
    
    if (userSpecifiedDelay) {
      
      timeout = setTimeout(endListener, userSpecifiedDelay);