错误：在 Flutter 中为此库禁用了空安全功能

如何解决错误：在 Flutter 中为此库禁用了空安全功能

我也在 pubspec 中添加了非 nullabe。我不想更改我的 SDK 版本，因为我已经将它用于其他依赖项，如果我更改版本会导致错误。请让我知道如何消除此错误。请让我知道如何解决这个问题，因为我是 Flutter 的新手

lib/BluetoothScanningDevices.dart:113:40: Error: Null safety features are disabled for this library.
Try removing the package language version or setting the language version to 2.12 or higher.
                children: snapshot.data!
                                       ^

我的 pubspec.yaml 文件如下所示：

name: epicare
description: Epilepsy monitoring application.

# The following line prevents the package from being accidentally published to
# pub.dev using `pub publish`. This is preferred for private packages.
publish_to: 'none' # Remove this line if you wish to publish to pub.dev

# The following defines the version and build number for your application.
# A version number is three numbers separated by dots,like 1.2.43
# followed by an optional build number separated by a +.
# Both the version and the builder number may be overridden in Flutter
# build by specifying --build-name and --build-number,respectively.
# In Android,build-name is used as versionName while build-number used as versionCode.
# Read more about Android versioning at https://developer.android.com/studio/publish/versioning
# In iOS,build-name is used as CFBundleShortVersionString while build-number used as CFBundLeversion.
# Read more about iOS versioning at
# https://developer.apple.com/library/archive/documentation/General/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html
version: 1.0.0+1

environment:
  sdk: ">=2.7.0 <3.0.0"
analyzer:
  enable-experiment:
    - non-nullable
dependencies:
  table_calendar: ^2.2.1
  shared_preferences: ^0.5.4+5
  permission_handler: ^5.0.0
  contacts_service: ^0.3.10
  url_launcher: ^5.4.2
  Flutter_switch: ^0.2.2
  Flutter:
    sdk: Flutter
  custom_switch: ^0.0.1
  charts_Flutter: ^0.10.0
  image_picker: ^0.6.7+22
  loading_animations: ^2.1.0
  Flutter_spinkit: ^4.1.2
  cupertino_icons: ^0.1.3
  Flutter_launcher_icons: ^0.9.0
  Flutter_local_notifications: ^1.4.4+2
  focused_menu: ^1.0.0
  Fluttertoast: ^4.0.0
  contact_picker: ^0.0.2
  google_sign_in: ^3.3.0
  http: ^0.12.0+4
  provider: ^5.0.0
  firebase_core: ^0.5.0
  firebase_auth: ^0.18.0+1
  Flutter_blue: ^0.8.0
  bluetooth_enable: ^0.1.1
  Flutter_bluetooth_serial: ^0.2.2

dev_dependencies:
  Flutter_test:
    sdk: Flutter

使用蓝牙功能的代码：

import 'package:Flutter/cupertino.dart';
import 'package:Flutter/material.dart';
import 'BluetoothConnectBand.dart';
import 'BluetoothConnectedSuccess.dart';
import 'package:Flutter_blue/Flutter_blue.dart';

class BluetoothScanningDevices extends StatefulWidget {
  @override
  _BluetoothScanningDevicesstate createState() =>
      _BluetoothScanningDevicesstate();
}

class _BluetoothScanningDevicesstate extends State<BluetoothScanningDevices> {
  FlutterBlue FlutterBlue = FlutterBlue.instance;

  @override
  Widget build(BuildContext context) {
    Size size = MediaQuery.of(context).size;
    return Scaffold(
      appBar: AppBar(
        backgroundColor: const Color(0xffE5E0A1),elevation: 0,centerTitle: true,title: Text(
          "Connect Band",style: TextStyle(
            fontSize: 15.0,color: Colors.black,fontFamily: 'Montserrat',fontWeight: FontWeight.normal,),leading: IconButton(
          icon: Icon(
            Icons.arrow_back,onpressed: () {
            Navigator.push(
              context,MaterialPageRoute(
                builder: (context) {
                  return BluetoothConnectBand();
                },);
          },actions: [
          Padding(
            padding: EdgeInsets.only(right: 5.0),child: FlatButton(
              onpressed: () {},child: Text(
                "Search",style: TextStyle(
                  fontSize: 15.0,)
        ],body: Column(
        crossAxisAlignment: CrossAxisAlignment.start,children: [
          Container(
              height: size.height * 0.4,width: size.width,color: const Color(0xffE5E0A1),child: Image.asset(
                'assets/images/bluetooth.png',)),Container(
            width: size.width,padding: EdgeInsets.symmetric(vertical: 20),child: Text(
              "Scanning Available Devices...",style: TextStyle(
                fontSize: 15.0,fontWeight: FontWeight.w400,textAlign: TextAlign.center,StreamBuilder<List<BluetoothDevice>>(
              stream: Stream.periodic(Duration(seconds: 2))
                  .asyncMap((_) => FlutterBlue.instance.connectedDevices),initialData: [],builder:  (c,snapshot) => Column(
                children: snapshot.data!
                    .map((d) => ListTile(
                  title: Text(d.name),subtitle: Text(d.id.toString()),trailing: StreamBuilder<BluetoothDeviceState>(
                    stream: d.state,initialData: BluetoothDeviceState.disconnected,builder: (c,snapshot) {
                      if (snapshot.data ==
                          BluetoothDeviceState.connected) {
                        return RaisedButton(
                          child: Text('OPEN'),onpressed: (){}
                        );
                      }
                      return Text(snapshot.data.toString());
                    },))
                    .toList(),],);
  }
}

解决方法

在空安全世界中，我们使用“！”表示该变量不能为空，但为了使用这些很酷的功能，dart SDK 版本应该大于 2.12。

错误信息明确指出了问题所在的行号，您无需更改版本或迁移，删除“！”在 BluetoothScanningDevices.dart 文件中第 113 行的 snapshot.data 之后。

我在代码片段中添加了一条注释，其中必须进行更改。

 builder:  (c,snapshot) => Column(
// Important: remove the exclamation after snapshot.data to fix the error.
                children: snapshot.data!
                    .map((d) => ListTile(
                  title: Text(d.name),subtitle: Text(d.id.toString()),trailing: StreamBuilder<BluetoothDeviceState>(
                    stream: d.state,initialData: BluetoothDeviceState.disconnected,builder: (c,snapshot) {
                      if (snapshot.data ==
                          BluetoothDeviceState.connected) {
                        return RaisedButton(
                          child: Text('OPEN'),onPressed: (){}
                        );
                      }
                      return Text(snapshot.data.toString());
                    },),))

有多种方法可以在遗留代码中使用不可为 null 的包，但它们需要使用 --no-sound-null-safety 调用各种命令。最好按照 https://dart.dev/null-safety/migration-guide 中所示的步骤迁移您的代码。

如何使用不同版本的相同包像这样使用 ->（环境上面的分析器）

web.php