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

);
      expect(EventPluginHub.__getListenerBank()).toEqual({});
    });
    it('should render composite components', function() {
      var Parent = React.createClass({
        render: function() {
          return <div><Child name="child" /></div>;
        },
      });
      var Child = React.createClass({
        render: function() {
          return <span>My name is {this.props.name}</span>;
        },
      });
      var response = ReactServerRendering.renderToString(
        <Parent />
      );
      expect(response).toMatch(
        '<div ' + ROOT_ATTRIBUTE_NAME + '="" ' +
          ID_ATTRIBUTE_NAME + '="[^"]+" ' +
          ReactMarkupChecksum.CHECKSUM_ATTR_NAME + '="[^"]+">' +
          '<span ' + ID_ATTRIBUTE_NAME + '="[^"]+">' +
            '<!-- react-text: [0-9]+ -->My name is <!-- /react-text -->' +
            '<!-- react-text: [0-9]+ -->child<!-- /react-text -->' +
          '</span>' +
        '</div>'
      );
    });
    it('should only execute certain lifecycle methods', function() {
      function runTest() {