我遇到了这个例子来获取小部件的大小/位置：https://medium.com/@diegoveloper/flutter-widget-size-and-position-b0a9ffed9407
我的代码有什么问题？
我得到错误：

The following NoSuchMethodError was thrown while handling a gesture:
I/flutter ( 8566): The method 'findRenderObject' was called on null.
I/flutter ( 8566): Receiver: null
I/flutter ( 8566): Tried calling: findRenderObject()

void main() {
      runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(home: MyWidgetSizeTMP());
  }
}

class MyWidgetSizeTMP extends StatefulWidget{
  @override
  MyWidgetSizeTMPState createState() {
    return new MyWidgetSizeTMPState();
  }
}

class MyWidgetSizeTMPState extends State<MyWidgetSizeTMP> {

  //creating Key for red panel
  GlobalKey _keyRed = GlobalKey();

  _getSizes() {
    final RenderBox renderBoxRed = _keyRed.currentContext.findRenderObject();
    final sizeRed = renderBoxRed.size;
    print("SIZE of Red: $sizeRed");
  }

  _getPositions() {
    final RenderBox renderBoxRed = _keyRed.currentContext.findRenderObject();
    final positionRed = renderBoxRed.localToGlobal(Offset.zero);
    print("POSITION of Red: $positionRed ");
  }

  _afterLayout(_) {
    _getSizes();
    _getPositions();
  }

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback(_afterLayout);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
      ),
      body: Column(
        children: <Widget>[
          Flexible(
            flex: 2,
            child: Container(
              color: Colors.red,
            ),
          ),
          Flexible(
            flex: 1,
            child: Container(
              color: Colors.purple,
            ),
          ),
          Flexible(
            flex: 3,
            child: Container(
              color: Colors.green,
            ),
          ),
          Spacer(),
          Padding(
            padding: const EdgeInsets.only(bottom: 8.0),
            child: Row(
              mainAxisAlignment: MainAxisAlignment.spaceAround,
              crossAxisAlignment: CrossAxisAlignment.center,
              children: <Widget>[
                MaterialButton(
                  elevation: 5.0,
                  padding: EdgeInsets.all(15.0),
                  color: Colors.grey,
                  child: Text("Get Sizes"),
                  onPressed: _getSizes,
                ),
                MaterialButton(
                  elevation: 5.0,
                  color: Colors.grey,
                  padding: EdgeInsets.all(15.0),
                  child: Text("Get Positions"),
                  onPressed: _getPositions,
                )
              ],
            ),
          )
        ],
      ),
    );
  }
}