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)

this.a = 1;
      this.b = 2;
    }
    var A = React.createClass({
      getInitialState: function() {
        return {};
      },
      render: function() {
        return <div />;
      },
    });
    var component = ReactTestUtils.renderIntoDocument(<A />);
    expect(() => component.forceUpdate('no')).toThrow(
      'forceUpdate(...): Expected the last optional `callback` argument ' +
      'to be a function. Instead received: string.'
    );
    expect(() => component.forceUpdate({})).toThrow(
      'forceUpdate(...): Expected the last optional `callback` argument ' +
      'to be a function. Instead received: Object.'
    );
    expect(() => component.forceUpdate(new Foo())).toThrow(
      'forceUpdate(...): Expected the last optional `callback` argument ' +
      'to be a function. Instead received: Foo (keys: a, b).'
    );
  });
});