如何在flutter的底部导航栏中添加一个垂直和水平居中的按钮？

Scaffold(
  appBar: AppBar(
    title: Text('Flutter 应用'),
  ),
  body: Center(
    child: Text('主页'),
  ),
  bottomNavigationBar: BottomNavigationBar(
    items: [
      BottomNavigationBarItem(
        icon: Icon(Icons.home),
        label: '主页',
      ),
      BottomNavigationBarItem(
        icon: Icon(Icons.search),
        label: '搜索',
      ),
      BottomNavigationBarItem(
        icon: Icon(Icons.person),
        label: '个人',
      ),
    ],
  ),
  floatingActionButton: Align(
    alignment: Alignment.center,
    child: FloatingActionButton(
      onPressed: () {
        // 点击按钮的回调函数
      },
      child: Icon(Icons.add),
    ),
  ),
);