**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

warning(
              false,
              'Use the `defaultValue` or `value` props instead of setting ' +
                'children on <textarea>.',
            );
          }
          invariant(
            defaultValue == null,
            'If you supply `defaultValue` on a <textarea>, do not pass children.',
          );
          if (Array.isArray(textareaChildren)) {
            invariant(
              textareaChildren.length <= 1,
              '<textarea> can only have at most one child.',
            );
            textareaChildren = textareaChildren[0];
          }
          defaultValue = '' + textareaChildren;
        }
        if (defaultValue == null) {
          defaultValue = '';
        }
        initialValue = defaultValue;
      }
      props = Object.assign({}, props, {
        value: undefined,
        children: '' + initialValue,
      });
    } else if (tag === 'select') {
      if (__DEV__) {