Chapter3.14:A value of type 'Scaffold?' can't be assigned to a variable of type 'Scaffold'.

[TheSparklingStarrySky]

      You can do this ：
      // 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 as Widget;