Failed to precompile build script/Ferry with Flutter

Question

I get this error after "flutter pub run build_runner build --delete-conflicting-outputs" , while I'm using ferry generator

[INFO] Generating build script...
[INFO] Generating build script completed, took 3.9s

[INFO] Precompiling build script......
[WARNING] /C:/src/flutter/.pub-cache/hosted/pub.dartlang.org/gql_code_build
er-0.3.0-alpha.0/lib/src/ast.dart:618:12: Error: A non-null value must be r
eturned since the return type 'Expression' doesn't allow null.
 - 'Expression' is from 'package:code_builder/src/specs/expression.dart' ('
/C:/src/flutter/.pub-cache/hosted/pub.dartlang.org/code_builder-4.1.0/lib/s
rc/specs/expression.dart').
Expression _directiveLocation(DirectiveLocation location) {
           ^
[INFO] Precompiling build script... completed, took 1m 19s

[SEVERE] Failed to precompile build script .dart_tool/build/entrypoint/buil
d.dart.
This is likely caused by a misconfigured builder definition.

pub finished with exit code 78