type 'UnspecifiedInvalidResult' is not a subtype of type 'LibraryElementResult' in type cast

**Steps to Reproduce**
run pub run build_runner build --delete-conflicting-outputs

**Code sample**
``` dart
import 'dart:core';
import 'package:hive/hive.dart';
part 'storedItem.g.dart';

@HiveType(typeId: 1)
class Person extends HiveObject {

@HiveField(0)
String name;

@HiveField(1)
int age;

Person({required this.name, required this.age});
}
```

[INFO] Generating build script completed, took 446ms
[INFO] Reading cached asset graph completed, took 54ms
[INFO] Checking for updates since last build completed, took 495ms
[SEVERE] hive_generator:hive_generator on lib/src/ local_storage/config.dart:

type 'UnspecifiedInvalidResult' is not a subtype of type 'LibraryElementResult' in type cast
[INFO] Running build completed, took 958ms
[INFO] Caching finalized dependency graph completed, took 28ms
[SEVERE] Failed after 1.0s

**Version**
- Platform: iOS, Android, Mac, Windows, Linux, Web
[✓] Flutter (Channel stable, 2.5.0, on macOS 11.0.1 20B29 darwin-x64, locale zh-Hans-CN)
• Flutter version 2.5.0 at /Users/zmtzawqlp/Documents/flutter/2.5.0
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision 4cc385b4b8 (5 weeks ago), 2021-09-07 23:01:49 -0700
• Engine revision f0826da7ef
• Dart version 2.14.0
• Pub download mirror https://pub.flutter-io.cn
• Flutter download mirror https://storage.flutter-io.cn
hive:
dependency: "direct main"
description:
name: hive
url: "https://pub.flutter-io.cn"
source: hosted
version: "2.0.4"
hive_generator:
dependency: "direct dev"
description:
name: hive_generator
url: "https://pub.flutter-io.cn"
source: hosted
version: "1.1.1"