BottomNavigationBarについて

BottomNavigationBar だと BottomNavigationBarItem の子が必須かもしれない。

https://github.com/flutter/flutter/issues/17099

ここの

 @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: body,
      bottomNavigationBar: new BottomAppBar(
          child: new Row(
            mainAxisSize: MainAxisSize.max,
            mainAxisAlignment: MainAxisAlignment.spaceAround,
            children: <Widget>[
              IconButton(
                  icon: Icon(Icons.home),
                  disabledColor: Colors.green,
                  onPressed: _currentIndex == 0
                      ? null
                      : () => setState(() => _currentIndex = 0)),
              IconButton(
                  icon: Icon(Icons.notifications),
                  disabledColor: Colors.green,
                  onPressed: _currentIndex == 1
                      ? null
                      : () => setState(() => _currentIndex = 1)),
              IconButton(
                  icon: Icon(Icons.settings),
                  disabledColor: Colors.green,
                  onPressed: _currentIndex == 2
                      ? null
                      : () => setState(() => _currentIndex = 2)),
            ],
          )
      ),//bottomAppBar
    );
  }

これならWidgetでいいかも？

これ参考になるかな？ Flutter Bottom Navigation Bar Android and IOS - Sarim Khan - Medium

また FlutterでiOS風のBottomNavigationBarを実装する - HeavenOSK - Medium

の bizz84さんのやつを見る？