Flutter-showBottomSheet底部彈出框

onPressed: () {
                    showModalBottomSheet(
                      context: context,
                      builder: (BuildContext context) {
                        return new Container(
                          height: 300.0,
                          child: Text('123'),
                        );
                      },
                    ).then((val) {
                      print(val);
                    });
                  },