Running setup below works without error, but then when running showOneTapSignOn I get the following error:

E AndroidRuntime: FATAL EXCEPTION: main
E AndroidRuntime: Process: com.auth.google, PID: 16379
E AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method 'android.app.ActivityThread$ApplicationThread android.app.ActivityThread.getApplicationThread()' on a null object reference
E AndroidRuntime: 	at android.app.Activity.startIntentSenderForResultInner(Activity.java:5970)
E AndroidRuntime: 	at android.app.Activity.startIntentSenderForResult(Activity.java:5941)
E AndroidRuntime: 	at androidx.activity.ComponentActivity.startIntentSenderForResult(ComponentActivity.java:772)
E AndroidRuntime: 	at androidx.core.app.ActivityCompat$Api16Impl.startIntentSenderForResult(ActivityCompat.java:816)
E AndroidRuntime: 	at androidx.core.app.ActivityCompat.startIntentSenderForResult(ActivityCompat.java:283)
E AndroidRuntime: 	at androidx.activity.ComponentActivity$2.onLaunch(ComponentActivity.java:228)
E AndroidRuntime: 	at androidx.activity.result.ActivityResultRegistry$2.launch(ActivityResultRegistry.java:175)
E AndroidRuntime: 	at androidx.activity.result.ActivityResultLauncher.launch(ActivityResultLauncher.java:47)
E AndroidRuntime: 	at com.auth.google.GoogleAuth$3.onSuccess(GoogleAuth.java:196)
E AndroidRuntime: 	at com.auth.google.GoogleAuth$3.onSuccess(GoogleAuth.java:191)
E AndroidRuntime: 	at com.google.android.gms.tasks.zzm.run(com.google.android.gms:play-services-tasks@@18.0.2:1)
E AndroidRuntime: 	at android.os.Handler.handleCallback(Handler.java:958)
E AndroidRuntime: 	at android.os.Handler.dispatchMessage(Handler.java:99)
E AndroidRuntime: 	at android.os.Looper.loopOnce(Looper.java:205)
E AndroidRuntime: 	at android.os.Looper.loop(Looper.java:294)
E AndroidRuntime: 	at android.app.ActivityThread.main(ActivityThread.java:8177)
E AndroidRuntime: 	at java.lang.reflect.Method.invoke(Native Method)
E AndroidRuntime: 	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:552)
E AndroidRuntime: 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:971)

C++ side:

GoogleAuth::GoogleAuth(QObject *parent)
    : QObject{parent}
{
QNativeInterface::QAndroidApplication::runOnAndroidMainThread([=]() {
        if (QJniObject::isClassAvailable("com/auth/google/GoogleAuth")) {
            qDebug() << "Found GoogleAuth Class";
            m_gauth = QJniObject("com/auth/google/GoogleAuth");
            if (!m_gauth.isValid()) {
                qDebug() << "m_gauth object not valid";
            } else {
                qDebug() << "m_gauth Valid";
                m_gauth.callMethod<void>("setup", QNativeInterface::QAndroidApplication::context());
            }
        } else {
            qDebug() << "Could not find GoogleAuth Class";
        }
    }).waitForFinished();
}

...

void GoogleAuth::Auth()
{
    QNativeInterface::QAndroidApplication::runOnAndroidMainThread([=]() {
        m_gauth.callMethod<void>("showOneTapSignOn", QNativeInterface::QAndroidApplication::context());
    }).waitForFinished();
}

Java side:

Note that I've altered the function below based on this video which says the Google docs uses a deprecated function: https://www.youtube.com/watch?v=Zz3412C4BSA

public class GoogleAuth extends AppCompatActivity {

    private SignInClient oneTapClient;
    private BeginSignInRequest signUpRequest;
    private ActivityResultLauncher<IntentSenderRequest> activityResultLauncher;
    private static final String TAG = "GoogleAuth";
    private static final int REQ_ONE_TAP = 2;

    public GoogleAuth() {
        Log.i(TAG, "GoogleAuth ctor running");
    }

    public void setup(Context qtContext) {
        if (qtContext == null) {
            Log.i(TAG, "qtContext == null");
        }
        oneTapClient = Identity.getSignInClient(qtContext);
        Log.i(TAG, "oneTapClient created");
        signUpRequest = BeginSignInRequest.builder()
                .setGoogleIdTokenRequestOptions(GoogleIdTokenRequestOptions.builder()
                        .setSupported(true)
                        // Your server's client ID, not your Android client ID.
                        .setServerClientId("<server client id>")
                        .setFilterByAuthorizedAccounts(false)
                        .build())
                .build();

        activityResultLauncher =
            registerForActivityResult(new ActivityResultContracts.StartIntentSenderForResult(),
            new ActivityResultCallback<ActivityResult>() {
                @Override
                public void onActivityResult(ActivityResult result) {
                    Log.i(TAG, "onActivityResult start");
                    if (result.getResultCode() == Activity.RESULT_OK) {
                        Log.i(TAG, "result.getResultCode() == Activity.RESULT_OK");
                        try {
                            SignInCredential credential = oneTapClient.getSignInCredentialFromIntent(result.getData());
                            String idToken = credential.getGoogleIdToken();
                            if (idToken != null) {
                                Log.i(TAG, "Got ID token");
                                // authenticate with backend server here!
                                String email = credential.getId();
                                Log.i(TAG, "Email = " + email);
                            }
                        } catch (ApiException e) {
                            Log.i(TAG, "ApiException!!");
                            e.printStackTrace();
                        }
                    }
                }
            });
    }

    public void showOneTapSignOn(Context qtContext) {
        Log.i(TAG, "showOneTapSignOnSTART!");
        oneTapClient.beginSignIn(signUpRequest)
            .addOnSuccessListener((Activity)qtContext, new OnSuccessListener<BeginSignInResult>() {
                @Override
                public void onSuccess(BeginSignInResult result) {
                    IntentSenderRequest intentSenderRequest =
                            new IntentSenderRequest.Builder(result.getPendingIntent().getIntentSender()).build();
                    activityResultLauncher.launch(intentSenderRequest);
                }
            })
            .addOnFailureListener((Activity)qtContext, new OnFailureListener() {
                @Override
                public void onFailure(@NonNull Exception e) {
                    // No Google Accounts found. Just continue presenting the signed-out UI.
                    Log.d(TAG, e.getLocalizedMessage());
                }
            });
        Log.i(TAG, "showOneTapSignOn END!");
    }
  }