我错过了什么？是因为飞镖版本还是其他东西，

environment:
  sdk: ">=2.16.1 <3.0.0"

1.错误

错误：无法从返回类型为“Widget”的函数返回值“null”，因为“Widget”不可为空。

• 'Widget'来自'pack： flutter/src/widget/frame.dart'

//显示警报对话框

showAlertDialog(BuildContext context) {
    AlertDialog alert = AlertDialog(
      content: Padding(
        padding: const EdgeInsets.only(top: 22.0, bottom: 22),
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
            Container(
              child: Text(
                "Would you like to get latest updates and notifications?",
                textAlign: TextAlign.center,
                style: TextStyle(color: Colors.black, fontSize: 18),
              ),
            ),
            SizedBox(height: 24),
            Row(mainAxisAlignment: MainAxisAlignment.center, children: <Widget>[
              FlatButton(
                onPressed: () {
                  setState(() {
                    snackBarText = "You will not recive notifications.";
                  });
                  Navigator.of(context).pop();
                  Navigator.of(context).pushReplacement(new MaterialPageRoute(
                      builder: (BuildContext context) => null));
                },
                child: Text(
                  "DENY",
                  style:
                      TextStyle(color: Colors.white, fontSize: 16, height: 1.2),
                ),
                color: Colors.purple,
              ),
              SizedBox(width: 10),
              FlatButton(
                onPressed: () {
                  setState(() {
                    snackBarText = "You will recive notifications.";
                  });
                  Navigator.of(context).pop();
                  Navigator.of(context).pushReplacement(new MaterialPageRoute(
                      builder: (BuildContext context) => null));
                },
                child: Text(
                  "ALLOW",
                  style: TextStyle(
                      color: Colors.purple, fontSize: 16, height: 1.2),
                ),
                shape: RoundedRectangleBorder(
                    side: BorderSide(width: 1, color: Colors.purple)),
                color: Colors.white,
              )
            ])
          ],
        ),
      ),
    );
    showDialog(
      context: context,
      builder: (BuildContext context) {
        return alert;
      },
    );
  }

**2.**错误

lib/ua_Screens. dart：29:8：错误：字段'_image'应该被初始化，因为它的类型'File'不允许null。

• 文件来自dart： io。文件_image; ^^^^^^

File _image;
Future get_image() async {
    final image = await ImagePicker.pickImage(source: ImageSource.camera);
    setState(() {
      _image = image;
    });
  }