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

expect(ReactDOM.findDOMNode(a).childNodes.length).toBe(2);
    expect(ReactDOM.findDOMNode(a).childNodes[0].id).toBe('three');
    expect(ReactDOM.findDOMNode(a).childNodes[1].id).toBe('two');
  });
  it('should work with no children', function() {
    ReactDOM.render(
      <ReactCSSTransitionGroup transitionName="yolo" />,
      container
    );
  });
  it('should work with a null child', function() {
    ReactDOM.render(
      <ReactCSSTransitionGroup transitionName="yolo">
        {[null]}
      </ReactCSSTransitionGroup>,
      container
    );
  });
  it('should transition from one to null', function() {
    var a = ReactDOM.render(
      <ReactCSSTransitionGroup transitionName="yolo">
        <span key="one" id="one" />
      </ReactCSSTransitionGroup>,
      container
    );
    expect(ReactDOM.findDOMNode(a).childNodes.length).toBe(1);
    ReactDOM.render(
      <ReactCSSTransitionGroup transitionName="yolo">
        {null}
      </ReactCSSTransitionGroup>,