flutter在生成g.dart文件时报错

[SEVERE] Failed to snapshot build script .dart_tool/build/entrypoint/build.dart.
This is likely caused by a misconfigured builder definition.
[SEVERE] .dart_tool/build/entrypoint/build.dart:23:3: Warning: 
Operand of null-aware operation '?.' has type 'SendPort' which
 excludes null. - 'SendPort' is from 'dart:isolate'.  
 sendPort?.send(result);  ^Error: Cannot run with sound null 
 safety, because the following dependenciesdon't support null 
safety: - package:build_runner_core - package:json_serializable - package:source_gen - package:build_config - package:build_runner - package:build - package:json_annotation 
- package:glob - package:crypto - package:logging - package:watcher - package:build_resolvers - package:timing - 
- package:graphs - package:package_config - package:yaml - package:analyzer - package:dart_style - package:checked_yaml - 
- package:pubspec_parse - package:build_daemon - package:args - 
- package:io - package:convert - package:pub_semver - package:_fe_analyzer_shared - package:http_multi_server - 
- package:shelf - package:stream_transform - package:http_parser - package:mime - package:shelf_web_socket - 
- package:web_socket_channel - package:cli_utilFor solutions, 
- see https://dart.dev/go/unsound-null-
- safety.dart_tool/build/entrypoint/build.dart:21:44: Error: The 
- parameter 'sendPort' can't have a value of 'null' because of its type 'SendPort', but the implicit default value is 'null'. 
- - 'SendPort' is from 'dart:isolate'.Try adding either an explicit non-'null' default value or the 'required' 
- modifier.void main(List<String> args, [_i5.SendPort 
- sendPort]) async {  

报这个错可能是版本的问题。

sdk: ">=2.12.0 <3.0.0" 改为   sdk: ">=2.8.0 <3.0.0"

相关依赖

dev_dependencies:
  cupertino_icons: ^1.0.2
  json_annotation: ^3.1.1
  build_runner: ^1.1.3
  json_serializable: ^3.5.1

改完之后更新下flutter

flutter packages upgrade

最后执行

flutter packages pub run build_runner build