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

this.setState({text: 'hello, world'});
        },
        render: function() {
          return <div>{this.state.text}</div>;
        },
      });
      ReactReconcileTransaction.prototype.perform = function() {
        
        throw new Error('Browser reconcile transaction should not be used');
      };
      var markup = ReactServerRendering.renderToString(
        <Component />
      );
      expect(markup.indexOf('hello, world') >= 0).toBe(true);
    });
  });
  it('warns with a no-op when an async setState is triggered', function() {
    class Foo extends React.Component {
      componentWillMount() {
        this.setState({text: 'hello'});
        setTimeout(() => {
          this.setState({text: 'error'});
        });
      }
      render() {
        return <div onClick={() => {}}>{this.state.text}</div>;
      }
    }
    spyOn(console, 'error');
    ReactServerRendering.renderToString(<Foo />);