
**Do you want to request a *feature* or report a *bug*?**
Bug

**What is the current behavior?**
The shallow renderer defaults a class component's state to empty object if it's not set. "Real" renderers don't seem to do this, so a component can render successfully in the shallow renderer, but fail at runtime in the browser with a null reference error.

**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem.**
Here is ReactDOM's behavior (crashes reading an uninitialized state value): https://codesandbox.io/s/4jv82zmp0
Here is the shallow renderer behavior (component able to render): https://codesandbox.io/s/pp733nrxzq

**What is the expected behavior?**
They initialize state in the same way.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
React 16. This wasn't an issue in React 15.
state = {update: false};
      shouldComponentUpdate(nextProps, nextState) {
        return this.state.update !== nextState.update;
      }
      render() {
        renderCounter++;
        return <div>{`${renderCounter}`}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<SimpleComponent />);
    expect(shallowRenderer.getRenderOutput()).toEqual(<div>1</div>);
    const instance = shallowRenderer.getMountedInstance();
    instance.setState({update: false});
    expect(shallowRenderer.getRenderOutput()).toEqual(<div>1</div>);
    instance.setState({update: true});
    expect(shallowRenderer.getRenderOutput()).toEqual(<div>2</div>);
  });
  it('should enable PureComponent to prevent a re-render', () => {
    let renderCounter = 0;
    class SimpleComponent extends React.PureComponent {