
**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.
invariant(
      React.isValidElement(element),
      'ReactShallowRenderer render(): Invalid component element.%s',
      typeof element === 'function'
        ? ' Instead of passing a component class, make sure to instantiate ' +
          'it by passing it to React.createElement.'
        : '',
    );
    
    invariant(
      typeof element.type !== 'string',
      'ReactShallowRenderer render(): Shallow rendering works only with custom ' +
        'components, not primitives (%s). Instead of calling `.render(el)` and ' +
        'inspecting the rendered output, look at `el.props` directly instead.',
      element.type,
    );
    invariant(
      typeof element.type === 'function',
      'ReactShallowRenderer render(): Shallow rendering works only with custom ' +
        'components, but the provided element type was `%s`.',
      Array.isArray(element.type)
        ? 'array'
        : element.type === null ? 'null' : typeof element.type,
    );
    if (this._rendering) {
      return;
    }
    this._rendering = true;
    this._element = element;
    this._context = context;