“AVOID using library private types in public APIs” – lint warning, even in in cookbook examples?
From the latest docs: Subclasses should override this method to return a newly created instance of their associated [State] subclass: @override State<MyWidget> createState() => _MyWidgetState(); So you should replace @override _AnimatedContainerAppState createState() => _AnimatedContainerAppState(); with @override State<AnimatedContainerApp> createState() => _AnimatedContainerAppState();