flutter Web) User model cannot be uploaded to firestore after signup

I'm going to upload the user model to the database after signing up as a member through firebase Auth on the flutter web. If sign up, I can register well with Firebase Auth. However, after that, an error is output while uploading to the database.

pubspec.uaml

    environment:
  sdk: ">=2.7.0 <3.0.0"

dependencies:
  flutter:
    sdk: flutter


  # The following adds the Cupertino Icons font to your application.
  # Use with the CupertinoIcons class for iOS style icons.
  cupertino_icons: ^1.0.2
  firebase_core: ^0.5.0 #웹을 위해 추가?
  firebase_auth: ^0.18.0 #0.16.1
  cloud_firestore: ^0.14.0 #0.13.7
  firebase_storage: ^4.0.0 #3.1.6
  flutter_web_frame: ^0.0.2 #웹에서 최대 사이즈를 지정해주는 패키지
  carousel_slider: ^3.0.0
  get: ^3.3.0
  cached_network_image: ^2.2.0+1
  provider: ^4.3.3

auth and firestore code

    class Transformers {
  //DocumentSnapshot -> UserModel Transform
  final toUser = StreamTransformer<DocumentSnapshot, UserModel>.fromHandlers(
    handleData: (snapshot, sink) async {
      sink.add(UserModel.fromSnapShot(snapshot));
    }
  );
}


class UserNetworkRepo with Transformers{
  //When signing up, check if user model is registered in the firestore and upload if not.
  Future<void> attempCreatUser({String userKey, String userEmail}) async {
    final DocumentReference userRef =
    FirebaseFirestore.instance.collection(COLLECTION_USERS).doc(userKey);

    //get user document
    DocumentSnapshot snapshot = await userRef.get();
    //check user database
    if (!snapshot.exists) {
      return await userRef.set(UserModel.getMapForCreatUser(userEmail));
    }
  }

  Stream<UserModel> getUserModelStream(String userKey) {
    return FirebaseFirestore.instance
        .collection(COLLECTION_USERS)
        .doc(userKey)
        .snapshots()
        //documentSnapshot data -> UserNodel
        .transform(toUser);
  }

During the debug, the StreamTransformer brakes and then outputs the following errors.

TypeError: Cannot read properties of null (reading 'Symbol(dartx._get)')
at user_model.UserModel.fromSnapShot.user_model.UserModel.fromMap (http://localhost:65386/packages/omd_test/model/user_model.dart.lib.js:103:42)
at new user_model.UserModel.fromSnapShot (http://localhost:65386/packages/omd_test/model/user_model.dart.lib.js:113:34)
at http://localhost:65386/packages/omd_test/repo/helper/transformer.dart.lib.js:36:18
at Generator.next (<anonymous>)
at runBody (http://localhost:65386/dart_sdk.js:40666:34)
at Object._async [as async] (http://localhost:65386/dart_sdk.js:40697:7)
at http://localhost:65386/packages/omd_test/repo/helper/transformer.dart.lib.js:35:137
at _HandlerEventSink.new.add (http://localhost:65386/dart_sdk.js:39044:11)
at _SinkTransformerStreamSubscription.new.[_handleData] (http://localhost:65386/dart_sdk.js:38869:34)
at _RootZone.runUnaryGuarded (http://localhost:65386/dart_sdk.js:40429:11)
at _ForwardingStreamSubscription.new.[_sendData] (http://localhost:65386/dart_sdk.js:33989:22)
at _ForwardingStreamSubscription.new.[_add] (http://localhost:65386/dart_sdk.js:33935:26)
at _ForwardingStreamSubscription.new.[_add] (http://localhost:65386/dart_sdk.js:38197:20)
at _MapStream.new.[_handleData] (http://localhost:65386/dart_sdk.js:38328:19)
at _ForwardingStreamSubscription.new.[_handleData] (http://localhost:65386/dart_sdk.js:38225:37)
at _RootZone.runUnaryGuarded (http://localhost:65386/dart_sdk.js:40429:11)
at _ForwardingStreamSubscription.new.[_sendData] (http://localhost:65386/dart_sdk.js:33989:22)
at _ForwardingStreamSubscription.new.[_add] (http://localhost:65386/dart_sdk.js:33935:26)
at _ForwardingStreamSubscription.new.[_add] (http://localhost:65386/dart_sdk.js:38197:20)
at _HandleErrorStream.new.[_handleData] (http://localhost:65386/dart_sdk.js:38403:19)
at _ForwardingStreamSubscription.new.[_handleData] (http://localhost:65386/dart_sdk.js:38225:37)
at _RootZone.runUnaryGuarded (http://localhost:65386/dart_sdk.js:40429:11)
at _ForwardingStreamSubscription.new.[_sendData] (http://localhost:65386/dart_sdk.js:33989:22)
at _ForwardingStreamSubscription.new.[_add] (http://localhost:65386/dart_sdk.js:33935:26)
at _ForwardingStreamSubscription.new.[_add] (http://localhost:65386/dart_sdk.js:38197:20)
at _MapStream.new.[_handleData] (http://localhost:65386/dart_sdk.js:38328:19)
at _ForwardingStreamSubscription.new.[_handleData] (http://localhost:65386/dart_sdk.js:38225:37)
at _RootZone.runUnaryGuarded (http://localhost:65386/dart_sdk.js:40429:11)
at _BroadcastSubscription.new.[_sendData] (http://localhost:65386/dart_sdk.js:33989:22)
at _BroadcastSubscription.new.[_add] (http://localhost:65386/dart_sdk.js:33935:26)
at _SyncBroadcastStreamController.new.[_sendData] (http://localhost:65386/dart_sdk.js:34655:34)
at _SyncBroadcastStreamController.new.add (http://localhost:65386/dart_sdk.js:34449:24)
at http://localhost:65386/packages/cloud_firestore_web/src/interop/utils/utils.dart.lib.js:367:22
at Object._checkAndCall (http://localhost:65386/dart_sdk.js:5586:16)
at Object.dcall (http://localhost:65386/dart_sdk.js:5591:17)
at Object.ret [as next] (http://localhost:65386/dart_sdk.js:62759:21)
at Object.next (https://www.gstatic.com/firebasejs/8.6.1/firebase-firestore.js:1:320206)
at next (https://www.gstatic.com/firebasejs/8.6.1/firebase-firestore.js:1:303665)
at https://www.gstatic.com/firebasejs/8.6.1/firebase-firestore.js:1:244953
[2021-11-02T07:24:18.188Z]  @firebase/firestore:

I'm making the web and the app together, the app works well. Can I know the problem with this?

Thank you.