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

bug

**What is the current behavior?**

The warning for a failure to initialize state when using `getDerivedStateFromProps` is not as clear as it could be.

This component produces the warning:
> MyComponent: Did not properly initialize state during construction. Expected state to be an object, but it was undefined.`

```js
class MyComponent extends React.Component {
  static getDerivedStateFromProps(nextProps, prevState) {
    return null;
  }
  render() {
    return <span>Foo</span>
  }
}
```

The warning can be resolved by adding `state = {}` either in the class definition or the constructor, or by not using `getDerivedStateFromProps`.

See https://github.com/reactjs/reactjs.org/issues/796 for more detail on why this warning message can be counter-intuitive.

Since the [warning is specific](https://github.com/facebook/react/blob/b548b3cd640dbd515f5d67dafc0216bb7ee0d796/packages/react-reconciler/src/ReactFiberClassComponent.js#L509-L510) to the use of `getDerivedStateFromProps` and not triggered by any other state or lifecycle methods, and state initialization isn't required for other state methods like `this.setState`, it would make sense to me to mention `getDerivedStateFromProps` explicitly in the warning.

Here's a proposal for a new warning message:
> MyComponent: Component state must be initialized when using getDerivedStateFromProps. Expected state to be an object, but it was undefined.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

React >= 16.3.0

children: nextChildren,
            childIndex: 0,
            context: context,
            footer: '',
          };
          if (__DEV__) {
            ((frame: any): FrameDev).debugElementStack = [];
          }
          this.stack.push(frame);
          return '';
        }
        // eslint-disable-next-line-no-fallthrough
        default:
          break;
      }
      if (typeof elementType === 'object' && elementType !== null) {
        switch (elementType.$$typeof) {
          case REACT_FORWARD_REF_TYPE: {
            const element: ReactElement = ((nextChild: any): ReactElement);
            const nextChildren = toArray(
              elementType.render(element.props, element.ref),
            );
            const frame: Frame = {
              type: null,
              domNamespace: parentNamespace,
              children: nextChildren,
              childIndex: 0,
              context: context,
              footer: '',
            };