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

},
          componentDidUpdate: function() {
            lifecycle.push('componentDidUpdate');
          },
          shouldComponentUpdate: function() {
            lifecycle.push('shouldComponentUpdate');
          },
          componentWillReceiveProps: function() {
            lifecycle.push('componentWillReceiveProps');
          },
          componentWillUnmount: function() {
            lifecycle.push('componentWillUnmount');
          },
        });
        var response = ReactServerRendering.renderToString(
          <TestComponent />
        );
        expect(response).toMatch(
          '<span ' + ROOT_ATTRIBUTE_NAME + '="" ' +
            ID_ATTRIBUTE_NAME + '="[^"]+" ' +
            ReactMarkupChecksum.CHECKSUM_ATTR_NAME + '="[^"]+">' +
            '<!-- react-text: [0-9]+ -->Component name: <!-- /react-text -->' +
            '<!-- react-text: [0-9]+ -->TestComponent<!-- /react-text -->' +
          '</span>'
        );
        expect(lifecycle).toEqual(
          ['getInitialState', 'componentWillMount', 'render']
        );
      }
      runTest();