**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

Component.prototype &&
          typeof Component.prototype.render === 'function'
        ) {
          const componentName = getComponentName(Component) || 'Unknown';
          if (!didWarnAboutBadClass[componentName]) {
            warningWithoutStack(
              false,
              "The <%s /> component appears to have a render method, but doesn't extend React.Component. " +
                'This is likely to cause errors. Change %s to extend React.Component instead.',
              componentName,
              componentName,
            );
            didWarnAboutBadClass[componentName] = true;
          }
        }
      }
      inst = Component(element.props, publicContext, updater);
      if (inst == null || inst.render == null) {
        child = inst;
        validateRenderResult(child, Component);
        return;
      }
    }
    inst.props = element.props;
    inst.context = publicContext;
    inst.updater = updater;
    let initialState = inst.state;
    if (initialState === undefined) {
      inst.state = initialState = null;