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(console.error.calls.count()).toBe(0);
  });
  it('should clean-up silently after the timeout elapses', function() {
    var a = ReactDOM.render(
      <ReactCSSTransitionGroup
        transitionName="yolo"
        transitionEnter={false}
        transitionLeaveTimeout={200}
      >
        <span key="one" id="one" />
      </ReactCSSTransitionGroup>,
      container
    );
    expect(ReactDOM.findDOMNode(a).childNodes.length).toBe(1);
    setTimeout.mock.calls.length = 0;
    ReactDOM.render(
      <ReactCSSTransitionGroup
        transitionName="yolo"
        transitionEnter={false}
        transitionLeaveTimeout={200}
      >
        <span key="two" id="two" />
      </ReactCSSTransitionGroup>,
      container
    );
    expect(ReactDOM.findDOMNode(a).childNodes.length).toBe(2);
    expect(ReactDOM.findDOMNode(a).childNodes[0].id).toBe('two');
    expect(ReactDOM.findDOMNode(a).childNodes[1].id).toBe('one');