Flutter coding issue - google sign in Authentication - Firebase -- 2

I'm encountering an issue with adding Google Sign-In authentication to a Flutter application. Specifically, I need help with the error handling for the authentication process. I've identified where the error is likely occurring within the code, but am stuck and could use some help. The project is for an Android platform. I'm hoping that with the help of an experienced freelancer, I can resolve this issue quickly and move ahead with the development process.