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

describe('ReactServerRendering', function() {
  beforeEach(function() {
    jest.resetModuleRegistry();
    React = require('React');
    ReactDOM = require('ReactDOM');
    ReactMarkupChecksum = require('ReactMarkupChecksum');
    ReactTestUtils = require('ReactTestUtils');
    ReactReconcileTransaction = require('ReactReconcileTransaction');
    ExecutionEnvironment = require('ExecutionEnvironment');
    ExecutionEnvironment.canUseDOM = false;
    ReactServerRendering = require('ReactServerRendering');
    var DOMProperty = require('DOMProperty');
    ID_ATTRIBUTE_NAME = DOMProperty.ID_ATTRIBUTE_NAME;
    ROOT_ATTRIBUTE_NAME = DOMProperty.ROOT_ATTRIBUTE_NAME;
  });
  describe('renderToString', function() {
    it('should generate simple markup', function() {
      var response = ReactServerRendering.renderToString(
        <span>hello world</span>
      );
      expect(response).toMatch(
        '<span ' + ROOT_ATTRIBUTE_NAME + '="" ' +
          ID_ATTRIBUTE_NAME + '="[^"]+" ' +
          ReactMarkupChecksum.CHECKSUM_ATTR_NAME + '="[^"]+">hello world</span>'
      );
    });
    it('should generate simple markup for self-closing tags', function() {
      var response = ReactServerRendering.renderToString(
        <img />
      );