stacked_firebase_auth-2.19.2 build error

```
stacked_firebase_auth-2.19.2/lib/src/firebase_authentication_service.dart:45:25:
Error: A value of type 'Future?' can't be returned from a function with return type 'Future?' because 'String?' is nullable and 'String' isn't.
- 'Future' is from 'dart:async'.
return firebaseAuth.currentUser?.getIdToken();
```
pubspec.yaml:

# Firebase
firebase_core: ^2.14.0
firebase_auth: ^4.6.3
cloud_firestore: ^4.8.2

# Authentication
stacked_firebase_auth: ^2.19.2

# Stacked State Management
stacked: ^3.4.1
stacked_services: ^1.2.0