
**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.
return <div>{`${this.state.count}`}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<SimpleComponent />);
    expect(scuCounter).toEqual(0);
    expect(shallowRenderer.getRenderOutput()).toEqual(<div>1</div>);
    
    const instance = shallowRenderer.getMountedInstance();
    instance.forceUpdate();
    expect(scuCounter).toEqual(0);
    expect(shallowRenderer.getRenderOutput()).toEqual(<div>1</div>);
    
    
    instance.setState(state => ({count: state.count + 1}));
    expect(scuCounter).toEqual(1);
    expect(instance.state.count).toEqual(2);
    expect(shallowRenderer.getRenderOutput()).toEqual(<div>1</div>);
    
    instance.forceUpdate();
    expect(scuCounter).toEqual(1);
    expect(instance.state.count).toEqual(2);
    expect(shallowRenderer.getRenderOutput()).toEqual(<div>2</div>);
  });
  it('should rerender when calling forceUpdate', () => {
    let renderCounter = 0;
    class SimpleComponent extends React.Component {