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

<ReactCSSTransitionGroup
        transitionName={customTransitionNames}
        transitionEnterTimeout={1}
        transitionLeaveTimeout={1}
      >
        <span key="one" id="one" />
      </ReactCSSTransitionGroup>,
      container
    );
    expect(ReactDOM.findDOMNode(a).childNodes.length).toBe(1);
    
    ReactDOM.render(
      <ReactCSSTransitionGroup
        transitionName={customTransitionNames}
        transitionEnterTimeout={1}
        transitionLeaveTimeout={1}
      >
        <span key="one" id="one" />
        <span key="two" id="two" />
      </ReactCSSTransitionGroup>,
      container
    );
    expect(ReactDOM.findDOMNode(a).childNodes.length).toBe(2);
    var enteringNode = ReactDOM.findDOMNode(a).childNodes[1];
    expect(CSSCore.hasClass(enteringNode, 'custom-entering')).toBe(true);
    
    ReactDOM.render(
      <ReactCSSTransitionGroup
        transitionName={customTransitionNames}
        transitionEnterTimeout={1}