Adding new field to Hive Object results in error from Adapter

**Question**
I've added 2 new fields to my existing Hive Object. I followed the rules, didn't change the HiveType and also the new fields have new HiveField numbers. I generated the new adapter using the command

`flutter packages pub run build_runner build`

The problem is that when I run my app and openBox is executed, the following exception is thrown:

`Unhandled Exception: type 'Null' is not a subtype of type 'String' in type cast`

I can see through console that the new fields are the problem, because the previous stored data in that box didn't have that fields. I read the documentation and there says that this is not supposed to happen.

**Code sample**
```dart
await Hive.initFlutter();
Hive.registerAdapter(ProductsAdapter());
await Hive.openBox("favorite_products"); // this throws the exception
```

**Version**
- Platform: iOS, Android
- Flutter version: 2.2.1
- Hive version: 2.0.4

Reproduce the failure from the provided Hive.initFlutter, ProductsAdapter registration, and Hive.openBox entry point using existing stored data. Inspect the generated ProductsAdapter and the field-reading path; done means an existing box opens after new fields are added, with coverage for previously stored data.

Written by the indexing model from the issue text.