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 updates = [];
    var Outer = React.createClass({
      getInitialState: function() {
        return {x: 0};
      },
      render: function() {
        updates.push('Outer-render-' + this.state.x);
        return <div><Inner x={this.state.x} ref="inner" /></div>;
      },
      componentDidUpdate: function() {
        var x = this.state.x;
        updates.push('Outer-didUpdate-' + x);
        updates.push('Inner-setState-' + x);
        this.refs.inner.setState({x: x}, function() {
          updates.push('Inner-callback-' + x);
        });
      },
    });
    var Inner = React.createClass({
      getInitialState: function() {
        return {x: 0};
      },
      render: function() {
        updates.push('Inner-render-' + this.props.x + '-' + this.state.x);
        return <div />;
      },
      componentDidUpdate: function() {
        updates.push('Inner-didUpdate-' + this.props.x + '-' + this.state.x);
      },
    });