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

transitionLeaveTimeout={1}
      >
        <span key="two" id="two" />
      </ReactCSSTransitionGroup>,
      container
    );
    expect(ReactDOM.findDOMNode(a).childNodes.length).toBe(2);
    var leavingNode = ReactDOM.findDOMNode(a).childNodes[0];
    expect(CSSCore.hasClass(leavingNode, 'custom-leaving')).toBe(true);
  });
  it('should clear transition timeouts when unmounted', function() {
    var Component = React.createClass({
      render: function() {
        return (
          <ReactCSSTransitionGroup
            transitionName="yolo"
            transitionEnterTimeout={500}>
            {this.props.children}
          </ReactCSSTransitionGroup>
        );
      },
    });
    ReactDOM.render(<Component/>, container);
    ReactDOM.render(<Component><span key="yolo" id="yolo"/></Component>, container);
    ReactDOM.unmountComponentAtNode(container);
    
    jest.runAllTimers();
  });
});