I am having trouble submitting the highscores to android leaderboards. Right now the leaderboards are empty and nothing is being submitted. I have an int "oldScore" that needs to be submitted. Right now I am trying a piece of code to submit it but the activity crashes when called. My code:
public class GameOver extends BaseGameActivity implements GoogleApiClient.ConnectionCallbacks, GoogleApiClient.OnConnectionFailedListener {
    private GoogleApiClient mGoogleApiClient;
    public static int score;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_game_over);
        mGoogleApiClient.connect();
        mGoogleApiClient = new GoogleApiClient.Builder(this)
        .addApi(Drive.API)
        .addScope(Drive.SCOPE_FILE)
        .addConnectionCallbacks(this)
        .addOnConnectionFailedListener(this)
        .build();
        int newScore = GameOver.score;
        SharedPreferences prefs = this.getSharedPreferences("myPrefsKey", Context.MODE_PRIVATE);
        int oldScore = prefs.getInt("key", 0);
        if (newScore > oldScore) {
            SharedPreferences.Editor edit = prefs.edit();
            edit.putInt("key", newScore);
            edit.commit();
            EditText HighScore = (EditText) findViewById(R.id.HighScoreT);
            HighScore.setText("" + newScore);
        } else {
            EditText HighScore = (EditText) findViewById(R.id.HighScoreT);
            HighScore.setText("" + oldScore);
            Games.Leaderboards.submitScoreImmediate(getApiClient(),String.valueOf(R.string.number_guesses_leaderboard), oldScore);
        }
    }
Logcat:
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'void com.google.android.gms.common.api.GoogleApiClient.connect()' on a null object reference