After updating HiveObject its key will become null

I think it's a bug, why is the key changing to null if it's must be constant? But I want to say that the key is not changing internally in the hive box, it's still the same, but HiveObject for some reason holds null after the update, in order to fix this I need to hot restart my app.

Reproducible code example with dependencies is provided below:

```dart
import 'package:flutter/material.dart';
import 'package:hive/hive.dart';
import 'package:path_provider/path_provider.dart';

part 'main.g.dart';

// dependencies:
// hive: ^2.0.5
// hive_flutter: ^1.1.0
// path_provider: ^2.0.8
// build_runner: ^2.1.7
// hive_generator: ^1.1.2

void main() async {
WidgetsFlutterBinding.ensureInitialized();

final directory = await getApplicationDocumentsDirectory();

Hive.init(directory.path);

Hive.registerAdapter(MyModelAdapter());

await Hive.openBox('my_model');

Hive.box('my_model').add(MyModel('Test'));

runApp(const MyApp());
}

class MyApp extends StatefulWidget {
const MyApp({Key? key}) : super(key: key);

@override
State createState() => _MyAppState();
}

class _MyAppState extends State {
late MyModel test;

@override
void initState() {
test = Hive.box('my_model').values.first;
super.initState();
}

@override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
body: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Row(),
// 2. Check how key will become null
// Accepted behaviour: key must not be null
Text(
'Model ${test.name} Key: ${test.key}',
),
// 1. Tap this button
TextButton(
onPressed: () {
Hive.box('my_model').put(
test.key,
MyModel('Test ${DateTime.now().second}'),
);
setState(() {});
},
child: const Text('update'),
),
],
),
),
);
}
}

@HiveType(typeId: 0)
class MyModel extends HiveObject {
MyModel(this.name);

@HiveField(0)
final String name;
}
```

Start with the provided Dart reproduction and trace the interaction between HiveObject.key and the box.put call that replaces the model. Confirm the key remains non-null after the update, and add or run a regression test covering this replacement behavior if the repository has a relevant test entry point.

Written by the indexing model from the issue text.