Flutter 中父容器顶部的 Gridview 间距额外区域

我正在创建一个洗牌游戏，为此，我设置了3个级别的网格，

我拿了一个300x300的固定大小的容器，我不想改变它的大小，

在这里，我想要把所有的网格按钮正确地放在一个网格内，但我在网格视图的顶部获得了空间，因此，按钮显示得不合适.

以下是我的代码

final kdecoration = BoxDecoration(
    border: Border.all(color: Colors.yellow, width: 1),
    borderRadius: BorderRadius.circular(5),
    color: Colors.red);

class HomeScreen extends StatefulWidget {
  const HomeScreen({Key? key}) : super(key: key);

  @override
  State<HomeScreen> createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  double w = 300;
  double h = 300;
  int gridvalue = 3;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        crossAxisAlignment: CrossAxisAlignment.center,
        children: [
          Center(
              child: Text(
            '$gridvalue x $gridvalue',
            style: TextStyle(fontSize: 30, color: Colors.blue),
          )),
          SizedBox(
            height: 20,
          ),
          Container(
              color: Colors.blue,
              height: h,
              width: w,
              child: GridView.builder(
                  physics: NeverScrollableScrollPhysics(),
                  itemCount: gridvalue * gridvalue,
                  gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
                    mainAxisSpacing: 0,
                    crossAxisSpacing: 0,
                    crossAxisCount: gridvalue,
                  ),
                  itemBuilder: (context, index) {
                    return Container(
                      decoration: kdecoration,
                      child: Center(
                          child: Text(
                        (index + 1).toString(),
                        style: TextStyle(fontSize: 20, color: Colors.white),
                      )),
                      //color: Colors.green,
                    );
                  })),
          SizedBox(
            height: 20,
          ),
          Container(
            child: Row(
              mainAxisAlignment: MainAxisAlignment.spaceEvenly,
              children: [
                ElevatedButton(
                    onPressed: () {
                      setState(() {
                        gridvalue = 3;
                      });
                    },
                    child: Text('3x3')),
                ElevatedButton(
                    onPressed: () {
                      setState(() {
                        gridvalue = 4;
                      });
                    },
                    child: Text('4x4')),
                ElevatedButton(
                    onPressed: () {
                      setState(() {
                        gridvalue = 5;
                      });
                    },
                    child: Text('5x5')),
              ],
            ),
          )
        ],
      ),
    );
  }
}

placed output image as well...