NullPointerException in in-App purchase code

Question

I am trying to implement the in-App purchase code in my android app. The idea is that when I press the button "botonComprar", it activates the purchase flow, but when I press it I get this error:

NullPointerException: Attempt to invoke virtual method 'void com.android.billingclient.api.BillingClient.startConnection(com.android.billingclient.api.BillingClientStateListener)' on a null object reference

This is my code, the error it's supposed to be on "establishConnection()" but I do not understand what is happening. Thank you in advanced.

BillingClient billingClient;
Prefs prefs;
ProductDetails productDetails;

//Button botonComprar = (Button) findViewById(R.id.botonEmpezar);

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

    PurchasesUpdatedListener purchasesUpdatedListener = new PurchasesUpdatedListener() {
        @Override
        public void onPurchasesUpdated(BillingResult billingResult, List<Purchase> purchases) {
            // To be implemented in a later section.
        }
    };

    BillingClient billingClient = BillingClient.newBuilder(this)
            .setListener(purchasesUpdatedListener)
            .enablePendingPurchases()
            .build();

    establishConnection();

}

void establishConnection() {
    billingClient.startConnection(new BillingClientStateListener() {
        @Override
        public void onBillingSetupFinished(BillingResult billingResult) {
            if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK) {
                // The BillingClient is ready. You can query purchases here.
                showProducts();
            }
        }

        @Override
        public void onBillingServiceDisconnected() {
            // Try to restart the connection on the next request to
            // Google Play by calling the startConnection() method.
            establishConnection();
        }
    });
}

void showProducts() {

    QueryProductDetailsParams queryProductDetailsParams =
            QueryProductDetailsParams.newBuilder()
                    .setProductList(
                            ImmutableList.of(
                                    QueryProductDetailsParams.Product.newBuilder()
                                            .setProductId("noanunciosymasextras")
                                            .setProductType(BillingClient.ProductType.SUBS)
                                            .build()))
                    .build();

    billingClient.queryProductDetailsAsync(
            queryProductDetailsParams,
            new ProductDetailsResponseListener() {
                public void onProductDetailsResponse(BillingResult billingResult,
                                                     List<ProductDetails> productDetailsList) {
                    // check billingResult
                    // process returned productDetailsList
                }
            }
    );

}

public void botonComprar (View view){

    BillingFlowParams billingFlowParams =
            BillingFlowParams.newBuilder()
                    .setProductDetailsParamsList(
                            ImmutableList.of(
                                    BillingFlowParams.ProductDetailsParams.newBuilder()
                                            .setProductDetails(productDetails)
                                            .build()
                            )
                    )
                    .build();

    billingClient.launchBillingFlow(this, billingFlowParams);

}

void handlePurchase(Purchase purchases) {

    if(!purchases.isAcknowledged()){
        billingClient.acknowledgePurchase(AcknowledgePurchaseParams
                .newBuilder()
                .setPurchaseToken(purchases.getPurchaseToken())
                .build(), billingResult -> {

            if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK) {
                //Setting setIsRemoveAd to true
                // true - No ads
                // false - showing ads.
                prefs.setIsRemoveAd(true);
                //  goBack();
            }
        });
    }
}



protected void onResume() {
    super.onResume();
    billingClient.queryPurchasesAsync(
            QueryPurchasesParams.newBuilder().setProductType(BillingClient.ProductType.INAPP).build(),
            (billingResult, list) -> {
                if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK) {
                    for (Purchase purchase : list) {
                        if (purchase.getPurchaseState() == Purchase.PurchaseState.PURCHASED && !purchase.isAcknowledged()) {
                            handlePurchase(purchase);
                        }
                    }
                }
            }
    );
}

Answer 1

Did not know how to solve it, and still don't. So I decided to start over and following this tutorial. I works perfectly.