The return type 'Widget?' isn't a 'Widget', as required by the closure's context

原代码

class ContextRoute extends StatelessWidget  {
  const ContextRoute();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Context测试"),
      ),
      body: Container(
        child: Builder(builder: (context) {
          // 在 widget 树中向上查找最近的父级`Scaffold`  widget
          Scaffold scaffold = context.findAncestorWidgetOfExactType<Scaffold>() as Scaffold;
          // 直接返回 AppBar的title， 此处实际上是Text("Context测试")
          return (scaffold.appBar as AppBar).title;
        }),
      ),
    );
  }
}

修改后

class ContextRoute extends StatelessWidget  {
  const ContextRoute();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Context测试"),
      ),
      body: Container(
        child: Builder(builder: (context) {
          // 在 widget 树中向上查找最近的父级`Scaffold`  widget
          Scaffold scaffold = context.findAncestorWidgetOfExactType<Scaffold>() as Scaffold;
          // 直接返回 AppBar的title， 此处实际上是Text("Context测试")
          return (scaffold.appBar as AppBar).title!;
        }),
      ),
    );
  }
}

在return (scaffold.appBar as AppBar).title后加了!（但是不知道为什么）
参考链接