مشکل در پرداخت درون برنامه بصورت مصرفی

سلام دوستان، وقت بخیر.

کسی می تونه کمک کنه مشکل این کد کجاس؟ وقتی روی Button کلیک می کنم. ارور Null Pointer Exception میده!

 public class BuyActivity extends Activity {

 
    private static final String TAG                    = "LOG";
    private static final String SKU_PREMIUM            = "500Coins";
    private boolean             mIsPremium             = false;
    private IabHelper           mHelper;
    public static final String  base64EncodedPublicKey = billing.rsaKey;
    protected Inventory         inventory;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);

        setContentView(R.layout.buy_board);

        Button btn500point = (Button) findViewById(R.id.btn500point);
              
        mHelper = new IabHelper(this, base64EncodedPublicKey);

        final IabHelper.QueryInventoryFinishedListener mGotInventoryListener = new IabHelper.QueryInventoryFinishedListener() {

            public void onQueryInventoryFinished(IabResult result, Inventory inventory) {
                Log.d(TAG, "Query inventory finished.");
                if (result.isFailure()) {
                    Log.d(TAG, "Failed to query inventory: " + result);
                    return;
                }
                else {
                    Log.d(TAG, "Query inventory was successful.");
                    // does the user have the premium upgrade?
                    mIsPremium = inventory.hasPurchase(SKU_PREMIUM);

                    // update UI accordingly

                    Log.d(TAG, "User is " + (mIsPremium ? "PREMIUM" : "NOT PREMIUM"));
                }

                Log.d(TAG, "Initial inventory query finished; enabling main UI.");
            }
        };

        final IabHelper.OnConsumeFinishedListener mConsumeFinishedListener =
                new IabHelper.OnConsumeFinishedListener() {

                    public void onConsumeFinished(Purchase purchase, IabResult result) {
                        if (result.isSuccess()) {
                            // provision the in-app purchase to the user
                            // (for example, credit 50 gold coins to player's character)
                        }
                        else {
                            // handle error
                        }
                    }
                };

        Log.d(TAG, "Starting setup.");
        mHelper.startSetup(new IabHelper.OnIabSetupFinishedListener() {

            public void onIabSetupFinished(IabResult result) {
                Log.d(TAG, "Setup finished.");

                if ( !result.isSuccess()) {
                    // Oh noes, there was a problem.
                    Log.d(TAG, "Problem setting up In-app Billing: " + result);
                }
                // Hooray, IAB is fully set up!
                mHelper.queryInventoryAsync(mGotInventoryListener);
            }
        });

        btn500point.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {
                mHelper.consumeAsync(inventory.getPurchase(billing.sku_500coins), mConsumeFinishedListener);
            }
        });

    }


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

        Log.d(TAG, "onActivityResult(" + requestCode + "," + resultCode + "," + data);

        // Pass on the activity result to the helper for handling
        if ( !mHelper.handleActivityResult(requestCode, resultCode, data)) {
            super.onActivityResult(requestCode, resultCode, data);
        } else {
            Log.d(TAG, "onActivityResult handled by IABUtil.");
        }
    }


    @Override
    public void onDestroy() {
        super.onDestroy();
        if (mHelper != null)
            mHelper.dispose();
        mHelper = null;
    }

}