clientId.secret cannot be null.. Error thrown null.

If I create a client id in dart code like this:
`final ClientId clientId = ClientId.serviceAccount(AppConstants.GOOGLE_API_CLIENT_ID);`

It crashes here:
```
Non-fatal Exception: io.flutter.plugins.firebase.crashlytics.FlutterError: Invalid argument(s): clientId.secret cannot be null.. Error thrown null.
at .refreshCredentials(.java:101)
at AutoRefreshingClient.send(AutoRefreshingClient.java:114)
at ApiRequester._request.simpleRequest(_request.java:220)
at ApiRequester._request(ApiRequester.java:263)
at ApiRequester.request(ApiRequester.java:69)
at AboutResource.get(AboutResource.java:141)
at GoogleDrive.upload(GoogleDrive.java:46)
```
Because clientId.secret is null, if I use serviceAccount. It works when I use ClientId(AppConstants.GOOGLE_API_CLIENT_ID, ''). Am I doing something wrong here? What is the point of ClientId.serviceAccount, if it cannot be used?