Calling `setState` in `componentWillMount` of a component causes the error

```
TypeError: Cannot read property '_currentElement' of null
    at ReactCompositeComponentMixin._updateRenderedComponent
```

when rendering server-side with `ReactDOMServer.renderToString` and preceded by a component that renders content using `ReactDOMServer.renderToString` or `ReactDOMServer.renderToStaticMarkup`.

Causes TypeError with 0.14.8 and 15.0.0-rc.2:
https://github.com/PKJedi/react-dom-server-test/blob/master/test.js

Works with 0.13:
https://github.com/PKJedi/react-dom-server-test/blob/master/0.13/test.js

ReactDOM.unmountComponentAtNode(element);
      expect(element.innerHTML).toEqual('');
      ReactDOM.render(<TestComponent name="x" />, element);
      expect(mountCount).toEqual(2);
      expect(element.innerHTML).not.toEqual(lastMarkup);
      
      
      
      
      ReactDOM.unmountComponentAtNode(element);
      expect(element.innerHTML).toEqual('');
      ExecutionEnvironment.canUseDOM = false;
      lastMarkup = ReactServerRendering.renderToString(
        <TestComponent name="x" />
      );
      ExecutionEnvironment.canUseDOM = true;
      element.innerHTML = lastMarkup;
      var instance = ReactDOM.render(<TestComponent name="x" />, element);
      expect(mountCount).toEqual(3);
      expect(element.innerHTML).toBe(lastMarkup);
      
      expect(numClicks).toEqual(0);
      ReactTestUtils.Simulate.click(ReactDOM.findDOMNode(instance.refs.span));
      expect(numClicks).toEqual(1);
      ReactDOM.unmountComponentAtNode(element);
      expect(element.innerHTML).toEqual('');
      
      
      element.innerHTML = lastMarkup;