안드로이드 스튜디오에서 구글 로그인 연동하는데 질문이 있습니다.

Question

오류도 안뜨고, 런타임에서 에러도 발생하지 않는데, 로그인이 안됩니다.

뭐가 문제일까요..?

public class MainActivity extends AppCompatActivity implements GoogleApiClient.OnConnectionFailedListener {

    private SignInButton btn_google;
    private FirebaseAuth mAuth;
    private GoogleApiClient googleApiClient;
    private static final int REQ_SIGN_GOOGLE=100;



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mAuth = FirebaseAuth.getInstance();


        GoogleSignInOptions googleSignInOptions = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                .requestIdToken(getString(R.string.default_web_client_id))
                .requestEmail()
                .build();

        googleApiClient = new GoogleApiClient.Builder(this)
        .enableAutoManage(  this, this )
        .addApi(Auth.GOOGLE_SIGN_IN_API, googleSignInOptions)
                .build();

        mAuth=FirebaseAuth.getInstance();

        btn_google = findViewById(R.id.btn_google);
        btn_google.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = Auth.GoogleSignInApi.getSignInIntent(googleApiClient);
                startActivityForResult(intent, REQ_SIGN_GOOGLE);

            }
        });
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if(requestCode==REQ_SIGN_GOOGLE){
            GoogleSignInResult result = Auth.GoogleSignInApi.getSignInResultFromIntent(data);
                    if(result.isSuccess()==true){
                        GoogleSignInAccount account = result.getSignInAccount();

                        resultLogin(account);
                    }
        }
    }

    private void resultLogin(final GoogleSignInAccount account) {
        AuthCredential credential = GoogleAuthProvider.getCredential(account.getIdToken(), null);
        mAuth.signInWithCredential(credential)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if(task.isSuccessful()){
                            Toast.makeText(MainActivity.this, "로그인 성공", Toast.LENGTH_SHORT).show();
                            Intent intent = new Intent(getApplicationContext(), resultActivity.class);
                            intent.putExtra("nickName", account.getDisplayName());
                            intent.putExtra("photoUrl", String.valueOf(account.getPhotoUrl()));


                            startActivity(intent);

                        }else {
                            Toast.makeText(MainActivity.this, "로그인 실패", Toast.LENGTH_SHORT).show();

                        }
                    }
                });
    }

    @Override
    public void onConnectionFailed(@NonNull ConnectionResult connectionResult) {

    }
}