值“null”；无法从返回类型为“Widget”的函数返回因为“小部件”不可为空

Question

我错过了什么？是因为dart版本还是其他原因，

environment:
  sdk: ">=2.16.1 <3.0.0"

1.错误

错误：无法从带有 return 的函数返回值“null” 输入“Widget”，因为“Widget”不可为空。

• “Widget”来自“package:flutter/src/widgets/framework.dart”

// 警报对话框显示

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：应初始化字段“_image” 因为它的类型“File”不允许为 null。

• “文件”来自“dart:io”。文件_图像； ^^^^^^

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

Answer 1

我认为你写了错误的代码，尝试下面的代码希望它对使用有所帮助。还有一件事不使用 FlatButton 使用 TextButton 因为 FlatButton 因抖动而贬值。

请参考 TextButton

您的警报对话框功能：

showAlertDialog(BuildContext context) {
    return 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(() {});
                  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(() {});
                  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,
              )
            ])
          ],
        ),
      ),
    );
  }

您的小部件：

ElevatedButton(
            onPressed: () {
              showDialog(
                context: context,
                builder: (BuildContext context) {
                  return showAlertDialog(context);
                },
              );
            },
            child: Text('Pressed Me'),
          ),

您的结果屏幕 ->

您的警报对话框 ->

您的第二个错误请参考我的答案此处和此处