[Flutter/Dart] Unhandled Exception: PlatformException(sign_in_failed, com.google.android.gms....

Phenomenon

When I try to sign in to Firebase with google_sign_in, it fails with the following error.

Unhandled Exception: PlatformException(sign_in_failed, com.google.android.gms.common.api.b: 12500: , null, null)

Cause

The following were the causes.

1 Firebase android app package name

In my case, in order to separate the development environment and the production environment on andorid, I added .debug to the app name in the debug build (Reference).

In this case, you also need to match the package name of the android app you add to FireBase.

(Project Overview>Project Settings>My Apps)

2 You have not registered a support email

Project Overview>Project Settings>Publish Settings

3 google-service.json is not updated

If you change any of the settings above (or other project settings), download and update google-service.json again.