revert(react): remove null union from state which is a BC (#26946)

types/create-react-class/create-react-class-tests.ts

@@ -56,7 +56,7 @@ const ClassicComponent: React.ClassicComponentClass<Props> = createReactClass<Pr
     shouldComponentUpdate(this: React.ClassicComponent<Props, State>, nextProps, nextState) {
         const newFoo: string = nextProps.foo;
         const newBar: number = nextState.bar;
-        return newFoo !== this.props.foo && newBar !== this.state!.bar;
+        return newFoo !== this.props.foo && newBar !== this.state.bar;
     },
     statics: {
         test: 1

types/react/index.d.ts

@@ -306,7 +306,7 @@ declare namespace React {
         // In the future, if we can define its call signature conditionally
         // on the existence of `children` in `P`, then we should remove this.
         readonly props: Readonly<{ children?: ReactNode }> & Readonly<P>;
-        state: null | Readonly<S>;
+        state: Readonly<S>;
         /**
          * @deprecated
          * https://reactjs.org/docs/legacy-context.html

types/react/test/index.ts

@@ -59,14 +59,7 @@ declare const container: Element;
         inputValue: string;
         seconds: number;
     }
-    /**
-     * This is a "pre EcmaScript class property era" style of setting state within constructor.
-     * This occurs also if you need to provide som logic before mounting your component.
-     * To mitigate this error you need to provide state property definition upfront.
-     */
     class SettingStateFromCtorComponent extends React.Component<Props, State, Snapshot> {
-        // uncomenting this fixes the error :)
-        // state: State;
         constructor(props: Props) {
             super(props);
             // $ExpectError
@@ -78,16 +71,17 @@ declare const container: Element;
     }
 
     class BadlyInitializedState extends React.Component<Props, State, Snapshot> {
-        // ExpectError -> this throws error on TS 2.6
+        // $ExpectError -> this throws error on TS 2.6 uncomment once TS requirement is TS >= 2.7
         // state = {
         //     secondz: 0,
         //     inputValuez: 'hello'
         // };
+        render() { return null; }
     }
     class BetterPropsAndStateChecksComponent extends React.Component<Props, State, Snapshot> {
         render() { return null; }
         componentDidMount() {
-            // $ExpectError
+            // $ExpectError -> this will be true in next BC release where state is gonna be `null | Readonly<S>`
             console.log(this.state.inputValue);
         }
         mutateState() {