Hello,

After updating to React 15.0.1, we are getting the following error:

```
Uncaught TypeError: Cannot read property 'addEventListener' of null
```

![image](https://cloud.githubusercontent.com/assets/4996164/14602577/5bb4e9f2-0571-11e6-9aa1-98de49f50d76.png)

ReactTestUtils.renderIntoDocument(<Top />);
    expect(numMiddleRenders).toBe(2);
    expect(numBottomRenders).toBe(1);
  });
  it('should flow updates correctly', function() {
    var willUpdates = [];
    var didUpdates = [];
    var UpdateLoggingMixin = {
      componentWillUpdate: function() {
        willUpdates.push(this.constructor.displayName);
      },
      componentDidUpdate: function() {
        didUpdates.push(this.constructor.displayName);
      },
    };
    var Box = React.createClass({
      mixins: [UpdateLoggingMixin],
      render: function() {
        return <div ref="boxDiv">{this.props.children}</div>;
      },
    });
    var Child = React.createClass({
      mixins: [UpdateLoggingMixin],
      render: function() {
        return <span ref="span">child</span>;
      },
    });
    var Switcher = React.createClass({
      mixins: [UpdateLoggingMixin],
      getInitialState: function() {