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

clearTimeout(timeout);
    });
  },
  componentWillAppear: function(done) {
    if (this.props.appear) {
      this.transition('appear', done, this.props.appearTimeout);
    } else {
      done();
    }
  },
  componentWillEnter: function(done) {
    if (this.props.enter) {
      this.transition('enter', done, this.props.enterTimeout);
    } else {
      done();
    }
  },
  componentWillLeave: function(done) {
    if (this.props.leave) {
      this.transition('leave', done, this.props.leaveTimeout);
    } else {
      done();
    }
  },
  render: function() {
    return onlyChild(this.props.children);
  },
});
module.exports = ReactCSSTransitionGroupChild;