**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

value: props.value != null ? props.value : props.defaultValue,
          checked: props.checked != null ? props.checked : props.defaultChecked,
        },
      );
    } else if (tag === 'textarea') {
      if (__DEV__) {
        ReactControlledValuePropTypes.checkPropTypes('textarea', props);
        if (
          props.value !== undefined &&
          props.defaultValue !== undefined &&
          !didWarnDefaultTextareaValue
        ) {
          warning(
            false,
            'Textarea elements must be either controlled or uncontrolled ' +
              '(specify either the value prop, or the defaultValue prop, but not ' +
              'both). Decide between using a controlled or uncontrolled textarea ' +
              'and remove one of these props. More info: ' +
              'https://fb.me/react-controlled-components',
          );
          didWarnDefaultTextareaValue = true;
        }
      }
      let initialValue = props.value;
      if (initialValue == null) {
        let defaultValue = props.defaultValue;
        
        let textareaChildren = props.children;
        if (textareaChildren != null) {
          if (__DEV__) {