
**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.
const shallowRenderer = createRenderer();
    shallowRenderer.render(<Component />);
    const mountedInstance = shallowRenderer.getMountedInstance();
    mountedInstance.setState({hasUpdatedState: true}, () => {
      expect(mockFn).toBeCalled();
      expect(mountedInstance.state.hasUpdatedState).toBe(true);
      done();
    });
  });
  it('throws usefully when rendering badly-typed elements', () => {
    const shallowRenderer = createRenderer();
    const renderAndVerifyWarningAndError = (Component, typeString) => {
      expect(() => {
        expect(() => shallowRenderer.render(<Component />)).toWarnDev(
          'React.createElement: type is invalid -- expected a string ' +
            '(for built-in components) or a class/function (for composite components) ' +
            `but got: ${typeString}.`,
        );
      }).toThrowError(
        'ReactShallowRenderer render(): Shallow rendering works only with custom ' +
          `components, but the provided element type was \`${typeString}\`.`,
      );
    };
    renderAndVerifyWarningAndError(undefined, 'undefined');
    renderAndVerifyWarningAndError(null, 'null');
    renderAndVerifyWarningAndError([], 'array');
    renderAndVerifyWarningAndError({}, 'object');
  });
});