原因：MaterialAppはルートに一つだけしか存在してはいけないというルールがあった

class View1 extends StatelessWidget {
  const View1({Key key}) : super(key: key);
 
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(title: Text('View2'),),
        body: Center(child: Text('View2'),),
        floatingActionButton: ElevatedButton(onPressed: () =>
            Navigator.push(context,
                MaterialPageRoute(builder: (context) => View2()
                )
            ),
        ),
      );
  }
}
 
class View2 extends StatelessWidget {
  const View2({Key key}) : super(key: key);
 
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('View2'),),
      body: Center(child: Text('View2'),),
    );
  }
}