**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

constructor() {
          super();
          this.state = state;
        }
        render() {
          return <span />;
        }
      }
      expect(() => test(<Foo />, 'SPAN', '')).toWarnDev(
        'Foo.state: must be set to an object or null',
        {withoutStack: true},
      );
    });
  });
  it('should render with null in the initial state property', () => {
    class Foo extends React.Component {
      constructor() {
        super();
        this.state = null;
      }
      render() {
        return <span />;
      }
    }
    test(<Foo />, 'SPAN', '');
  });
  it('setState through an event handler', () => {
    class Foo extends React.Component {