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

}
/**
 * Render a ReactElement to its initial HTML. This should only be used on the
 * server.
 * See https://facebook.github.io/react/docs/top-level-api.html#reactdomserver.rendertostring
 */
function renderToString(element) {
  invariant(
    ReactElement.isValidElement(element),
    'renderToString(): You must pass a valid ReactElement.'
  );
  return renderToStringImpl(element, false);
}
/**
 * Similar to renderToString, except this doesn't create extra DOM attributes
 * such as data-react-id that React uses internally.
 * See https://facebook.github.io/react/docs/top-level-api.html#reactdomserver.rendertostaticmarkup
 */
function renderToStaticMarkup(element) {
  invariant(
    ReactElement.isValidElement(element),
    'renderToStaticMarkup(): You must pass a valid ReactElement.'
  );
  return renderToStringImpl(element, true);
}
module.exports = {
  renderToString: renderToString,
  renderToStaticMarkup: renderToStaticMarkup,
};