**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

if (typeof Component.getDerivedStateFromProps !== 'function') {
          inst.componentWillMount();
        }
      }
      if (
        typeof inst.UNSAFE_componentWillMount === 'function' &&
        typeof Component.getDerivedStateFromProps !== 'function'
      ) {
        
        
        inst.UNSAFE_componentWillMount();
      }
      if (queue.length) {
        let oldQueue = queue;
        let oldReplace = replace;
        queue = null;
        replace = false;
        if (oldReplace && oldQueue.length === 1) {
          inst.state = oldQueue[0];
        } else {
          let nextState = oldReplace ? oldQueue[0] : inst.state;
          let dontMutate = true;
          for (let i = oldReplace ? 1 : 0; i < oldQueue.length; i++) {
            let partial = oldQueue[i];
            let partialState =
              typeof partial === 'function'
                ? partial.call(inst, nextState, element.props, publicContext)
                : partial;
            if (partialState != null) {
              if (dontMutate) {