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)

var instance = ReactTestUtils.renderIntoDocument(<Outer />);
    updates.push('Outer-setState-1');
    instance.setState({x: 1}, function() {
      updates.push('Outer-callback-1');
      updates.push('Outer-setState-2');
      instance.setState({x: 2}, function() {
        updates.push('Outer-callback-2');
      });
    });
    
    expect(updates).toEqual([
      'Outer-render-0',
        'Inner-render-0-0',
      'Outer-setState-1',
        'Outer-render-1',
          'Inner-render-1-0',
          'Inner-didUpdate-1-0',
        'Outer-didUpdate-1',
          'Inner-setState-1',
            'Inner-render-1-1',
            'Inner-didUpdate-1-1',
          'Inner-callback-1',
      'Outer-callback-1',
      'Outer-setState-2',
        'Outer-render-2',
          'Inner-render-2-1',
          'Inner-didUpdate-2-1',
        'Outer-didUpdate-2',
          'Inner-setState-2',
            'Inner-render-2-2',