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

var transaction;
  try {
    ReactUpdates.injection.injectBatchingStrategy(ReactServerBatchingStrategy);
    transaction = ReactServerRenderingTransaction.getPooled(makeStaticMarkup);
    return transaction.perform(function() {
      var componentInstance = instantiateReactComponent(element, true);
      var markup = ReactReconciler.mountComponent(
        componentInstance,
        transaction,
        null,
        ReactDOMContainerInfo(),
        emptyObject
      );
      if (__DEV__) {
        ReactInstrumentation.debugTool.onUnmountComponent(
          componentInstance._debugID
        );
      }
      if (!makeStaticMarkup) {
        markup = ReactMarkupChecksum.addChecksumToMarkup(markup);
      }
      return markup;
    }, null);
  } finally {
    ReactServerRenderingTransaction.release(transaction);
    
    
    ReactUpdates.injection.injectBatchingStrategy(ReactDefaultBatchingStrategy);
  }