在字符偏移量 null 处编译 null 时崩溃 - Flutter 和 Dart

Question

我是 flutter 和 dart 的新手，我正在尝试克隆 Instagram 以用于学习目的，但我收到以下错误。

你能帮我找出代码哪里不正确吗？

最后输出消息错误。

我尝试谷歌，并重构代码以应用空安全，但实际上我不知道错误的代码到底在哪里。

主程序.dart

import 'package:flutter/material.dart';
import 'package:instagram_flutter/responsive/mobile_screen_layout.dart';
import 'package:instagram_flutter/responsive/responsive_layout_screen.dart';
import 'package:instagram_flutter/responsive/web_screen_layout.dart';
import 'package:instagram_flutter/utils/colors.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Instagram Clone',
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: mobileBackgroundColor,
      ),
      home: ResponsiveLayout(
        webScreenLayout: WebScreenLayout(),
        mobileScreenLayout: MobileScreenLayout(),
      ),
    );
  }
}

mobile_screen_layout.dart

import 'package:flutter/material.dart';

class MobileScreenLayout extends StatelessWidget {
  const MobileScreenLayout({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Text('This is mobile'),
      ),
    );
  }
}

响应式布局屏幕.dart

在下面的 IF ELSE 中，我尝试以某种方式进行验证，以验证该值是否为零或 null，但没有成功。我也尝试把？在 contrainsts.maxWidth 的末尾，因为我在研究中看到这避免了空值，但也不起作用。

import 'package:flutter/material.dart';
import 'package:instagram_flutter/responsive/web_screen_layout.dart';
import 'package:instagram_flutter/utils/dimensions.dart';

class ResponsiveLayout extends StatelessWidget {
  final Widget webScreenLayout;
  final Widget mobileScreenLayout;

  const ResponsiveLayout({
    super.key,
    required this.webScreenLayout,
    required this.mobileScreenLayout,
  });

  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
      builder: (context, constraints) {
        if (constraints.maxWidth > webScreenSize) {
          return webScreenLayout;
        }
        return mobileScreenLayout;
      },
    );
  }
}

web_screen_layout.dart

import 'package:flutter/material.dart';

class WebScreenLayout extends StatelessWidget {
  const WebScreenLayout({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Text('This is web'),
      ),
    );
  }
}

颜色.dart

import 'package:flutter/material.dart';

const mobileBackgroundColor = Color.fromRGBO(0, 0, 0, 1);
const webBackgroundColor = Color.fromRGBO(18, 18, 18, 1);
const mobileSearchColor = Color.fromRGBO(38, 38, 38, 1);
const blueColor = Color.fromRGBO(0, 149, 246, 1);
const primaryColor = Colors.white;
const secondaryColor = Colors.grey;

维度.dart

import 'package:flutter/material.dart';
import 'package:instagram_flutter/responsive/web_screen_layout.dart';

const webScreenSize = 600;

输出

[Running] dart "c:\Development\instagram_flutter\lib\main.dart"
Crash when compiling null,
at character offset null:
Null check operator used on a null value
#0      InferableTypeBuilderMixin.type (package:front_end/src/fasta/builder/type_builder.dart:392:29)
#1      InferableTypeBuilder.inferType (package:front_end/src/fasta/builder/omitted_type_builder.dart:155:12)
#2      SourceLoader.performTopLevelInference (package:front_end/src/fasta/source/source_loader.dart:2358:19)
#3      KernelTarget.buildOutlines.<anonymous closure> (package:front_end/src/fasta/kernel/kernel_target.dart:532:14)
<asynchronous suspension>
#4      withCrashReporting (package:front_end/src/fasta/crash.dart:122:12)
<asynchronous suspension>
#5      _buildInternal (package:front_end/src/kernel_generator_impl.dart:139:7)
<asynchronous suspension>
#6      withCrashReporting (package:front_end/src/fasta/crash.dart:122:12)
<asynchronous suspension>
#7      generateKernel.<anonymous closure> (package:front_end/src/kernel_generator_impl.dart:47:12)
<asynchronous suspension>
#8      generateKernel (package:front_end/src/kernel_generator_impl.dart:46:10)
<asynchronous suspension>
#9      kernelForModule (package:front_end/src/api_prototype/kernel_generator.dart:100:11)
<asynchronous suspension>
#10     SingleShotCompilerWrapper.compileInternal (file:///C:/b/s/w/ir/cache/builder/sdk/pkg/vm/bin/kernel_service.dart:412:11)
<asynchronous suspension>
#11     Compiler.compile.<anonymous closure> (file:///C:/b/s/w/ir/cache/builder/sdk/pkg/vm/bin/kernel_service.dart:221:45)
<asynchronous suspension>
#12     _processLoadRequest (file:///C:/b/s/w/ir/cache/builder/sdk/pkg/vm/bin/kernel_service.dart:914:37)
<asynchronous suspension>


#0      InferableTypeBuilderMixin.type (package:front_end/src/fasta/builder/type_builder.dart:392:29)
#1      InferableTypeBuilder.inferType (package:front_end/src/fasta/builder/omitted_type_builder.dart:155:12)
#2      SourceLoader.performTopLevelInference (package:front_end/src/fasta/source/source_loader.dart:2358:19)
#3      KernelTarget.buildOutlines.<anonymous closure> (package:front_end/src/fasta/kernel/kernel_target.dart:532:14)
<asynchronous suspension>
#4      withCrashReporting (package:front_end/src/fasta/crash.dart:122:12)
<asynchronous suspension>
#5      _buildInternal (package:front_end/src/kernel_generator_impl.dart:139:7)
<asynchronous suspension>
#6      withCrashReporting (package:front_end/src/fasta/crash.dart:122:12)
<asynchronous suspension>
#7      generateKernel.<anonymous closure> (package:front_end/src/kernel_generator_impl.dart:47:12)
<asynchronous suspension>
#8      generateKernel (package:front_end/src/kernel_generator_impl.dart:46:10)
<asynchronous suspension>
#9      kernelForModule (package:front_end/src/api_prototype/kernel_generator.dart:100:11)
<asynchronous suspension>
#10     SingleShotCompilerWrapper.compileInternal (file:///C:/b/s/w/ir/cache/builder/sdk/pkg/vm/bin/kernel_service.dart:412:11)
<asynchronous suspension>
#11     Compiler.compile.<anonymous closure> (file:///C:/b/s/w/ir/cache/builder/sdk/pkg/vm/bin/kernel_service.dart:221:45)
<asynchronous suspension>
#12     _processLoadRequest (file:///C:/b/s/w/ir/cache/builder/sdk/pkg/vm/bin/kernel_service.dart:914:37)
<asynchronous suspension>


[Done] exited with code=252 in 5.039 seconds

Answer 1

问题来自ResponsiveLayout

if (constraints.maxWidth > webScreenSize?) { //here
  return webScreenLayout;
}

删除?并喜欢

if (constraints.maxWidth > webScreenSize) {  
  return webScreenLayout;
}