JsonSyntaxException в интеграции платежей Stripe

Question

Я пытаюсь использовать полосу в качестве платежного шлюза для моего приложения firebase android. Когда я запускаю свое приложение для проверки всего, мое приложение автоматически вылетает при запуске. Я хотел бы знать, что означает эта ошибка и как я могу решить эту проблему. Ниже я предоставил сообщение об ошибке вместе с моим кодом. Заранее спасибо.

Это ошибка, которую я получаю

FATAL EXCEPTION: OkHttp Dispatcher
    Process: com.myapp.aap, PID: 30839
    com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 1 path $

Вот мой код -

// Configure the SDK with your Stripe publishable key so it can make requests to Stripe
        stripe = new Stripe(
                getApplicationContext(),
                Objects.requireNonNull("pk_test_51GzvqSI4qFEpgfxS15GlPnxcrL4Sb5CZsv5AWXGPW1ihpQBTB9OPSh9nnZxY2zY71bVacdmJQ4gRxKYmCw3SCVQo00KUZF8psk")
        );
        startCheckout();
    private void startCheckout(){
        // Create a PaymentIntent by calling the server's endpoint.
        MediaType mediaType = MediaType.get("application/json; charset=utf-8");
        String json = "";
        RequestBody body = RequestBody.create(json, mediaType);
        Request request = new Request.Builder()
                .url(BACKEND_URL + "create-payment-intent.json")
                .post(body)
                .build();
        httpClient.newCall(request)
                .enqueue(new PayCallback(PaymentPageActivity.this));


        // Hook up the pay button to the card widget and stripe instance
        Button payButton = findViewById(R.id.payButton);
        payButton.setOnClickListener((View view) -> {
            CardInputWidget cardInputWidget = findViewById(R.id.cardInputWidget);
            PaymentMethodCreateParams params = cardInputWidget.getPaymentMethodCreateParams();
            if (params != null) {
                ConfirmPaymentIntentParams confirmParams = ConfirmPaymentIntentParams
                        .createWithPaymentMethodCreateParams(params, paymentIntentClientSecret);
                stripe.confirmPayment(PaymentPageActivity.this, confirmParams);
            }
        });
    }

    private void displayAlert(@NonNull String title,
                              @Nullable String message) {
        AlertDialog.Builder builder = new AlertDialog.Builder(this)
                .setTitle(title)
                .setMessage(message);
        builder.setPositiveButton("Ok", null);
        builder.create().show();
    }
    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        // Handle the result of stripe.confirmPayment
        stripe.onPaymentResult(requestCode, data, new PaymentResultCallback(this));
    }
    private void onPaymentSuccess(@NonNull final Response response) throws IOException {
        Gson gson = new Gson();
        Type type = new TypeToken<Map<String, String>>(){}.getType();
        Map<String, String> responseMap = gson.fromJson(
                Objects.requireNonNull(response.body()).string(),
                type
        );
        paymentIntentClientSecret = responseMap.get("clientSecret");
    }
    private static final class PayCallback implements Callback {
        @NonNull private final WeakReference<PaymentPageActivity> activityRef;
        PayCallback(@NonNull PaymentPageActivity activity) {
            activityRef = new WeakReference<>(activity);
        }
        @Override
        public void onFailure(@NonNull Call call, @NonNull IOException e) {
            final PaymentPageActivity activity = activityRef.get();
            if (activity == null) {
                return;
            }
            activity.runOnUiThread(() ->
                    Toast.makeText(
                            activity, "Error: " + e.toString(), Toast.LENGTH_LONG
                    ).show()
            );
        }
        @Override
        public void onResponse(@NonNull Call call, @NonNull final Response response)
                throws IOException {
            final PaymentPageActivity activity = activityRef.get();
            if (activity == null) {
                return;
            }
            if (!response.isSuccessful()) {
                activity.runOnUiThread(() ->
                        Toast.makeText(
                                activity, "Error: " + response.toString(), Toast.LENGTH_LONG
                        ).show()
                );
            } else {
                activity.onPaymentSuccess(response);
            }
        }
    }
    private static final class PaymentResultCallback
            implements ApiResultCallback<PaymentIntentResult> {
        @NonNull private final WeakReference<PaymentPageActivity> activityRef;
        PaymentResultCallback(@NonNull PaymentPageActivity activity) {
            activityRef = new WeakReference<>(activity);
        }
        @Override
        public void onSuccess(@NonNull PaymentIntentResult result) {
            final PaymentPageActivity activity = activityRef.get();
            if (activity == null) {
                return;
            }
            PaymentIntent paymentIntent = result.getIntent();
            PaymentIntent.Status status = paymentIntent.getStatus();
            if (status == PaymentIntent.Status.Succeeded) {
                // Payment completed successfully
                Gson gson = new GsonBuilder().setPrettyPrinting().create();
                activity.displayAlert(
                        "Payment completed",
                        gson.toJson(paymentIntent)
                );
            } else if (status == PaymentIntent.Status.RequiresPaymentMethod) {
                // Payment failed – allow retrying using a different payment method
                activity.displayAlert(
                        "Payment failed",
                        Objects.requireNonNull(paymentIntent.getLastPaymentError()).getMessage()
                );
            }
        }
        @Override
        public void onError(@NonNull Exception e) {
            final PaymentPageActivity activity = activityRef.get();
            if (activity == null) {
                return;
            }
            // Payment request failed – allow retrying using the same payment method
            activity.displayAlert("Error", e.toString());
        }
    }