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 = require('ReactCSSTransitionGroup');
    container = document.createElement('div');
    spyOn(console, 'error');
  });
  it('should warn if timeouts aren\'t specified', function() {
    ReactDOM.render(
      <ReactCSSTransitionGroup
        transitionName="yolo"
        transitionEnter={false}
        transitionLeave={true}
      >
        <span key="one" id="one" />
      </ReactCSSTransitionGroup>,
      container
    );
    
    expect(console.error.calls.count()).toBe(1);
  });
  it('should not warn if timeouts is zero', function() {
    ReactDOM.render(
      <ReactCSSTransitionGroup
        transitionName="yolo"
        transitionEnter={false}
        transitionLeave={true}
        transitionLeaveTimeout={0}
      >
        <span key="one" id="one" />
      </ReactCSSTransitionGroup>,
      container
    );