Error al llamar a una función de un contrato inteligente con web3j

Estoy tratando de usar una función que devuelve un valor del contrato inteligente (pakoin).

contract Pacoin {
    /* This creates an array with all balances */
    mapping (address => uint256) public balanceOf;
    mapping (address => uint256) public vales;
    string public name;
    string public symbol;
    uint8 public decimals;
    address public pakos;

    //Here some functions

    function nPakoins() public constant returns (uint256 npakoins) {
        npakoins = balanceOf[msg.sender];
    }

    function nVales() public constant returns (uint256 nvales) {
        nvales = vales[msg.sender];
    }

}

A pesar de intentarlo de diferentes formas, me dan muchos errores y no sé cómo solucionarlos.

BigInteger balance = pakoin.nPakoins(publicKeyUser).send();

Sin embargo, en lugar de devolver el valor, la aplicación se cierra repentinamente y tengo el siguiente error en la consola:

en clase Ljava/lang/reflect/Type; o sus superclases (la declaración de 'java.lang.reflect.Type' aparece en /system/framework/core-oj.jar) en org.web3j.abi.TypeReference.getClassType(TypeReference.java:54) en org.web3j .abi.FunctionReturnDecoder.build(FunctionReturnDecoder.java:99) en org.web3j.abi.FunctionReturnDecoder.decode(FunctionReturnDecoder.java:42) en org.web3j.tx.Contract.executeCall(Contract.java:159) en org. web3j.tx.Contract.executeCallSingleValueReturn(Contract.java:165) en org.web3j.tx.Contract.executeCallSingleValueReturn(Contract.java:176) en org.web3j.tx.Contract.lambda$executeRemoteCallSingleValueReturn$1$Contract(Contract.java :232) en org.web3j.tx.Contract$$Lambda$1.call(Fuente desconocida) en org.web3j.protocol.core.RemoteCall.send(RemoteCall.java:30) en com.daedasan.sgssi_pakos.MainActivity$ReadTask .

P: ¿ Sabe cómo resolver este problema u otra forma de hacerlo? ¡Gracias de antemano!

Aquí está toda la clase.

public class MainActivity extends AppCompatActivity {

    private final static String publicKeyUser = "0x4B5867c957a0950d4086172cA215Ac432b28b868";
    private final static String privateKeyRinkeby = "***MyPrivateKey***";
    private final static String greeterContractAddressRinkeby = "0x7a698A3072C01c4a9339ea929Cd2a241bED81937";
    private final static String rinkebyUrl = "https://rinkeby.infura.io/hbvHwdGroO3GR4JQJvEl";
    //private static final Logger log = LoggerFactory.getLogger(MainActivity.class);
    private ProgressBar progressBar;
    private ProgressBar progressBar2;
    private EditText editText;
    private TextView comprarTextView;
    private TextView gasPriceTextView;
    private TextView gasLimitTextView;
    private SeekBar gasPriceSeekBar;
    private Button botonNumPakoins;
    private EditText nVales;

    private Web3j web3j;

    private Credentials credentials = Credentials.create(privateKeyRinkeby);
    private int minimumGasLimit = 50000;
    private BigInteger gasLimit = new BigInteger(String.valueOf(minimumGasLimit));

    public MainActivity() throws IOException, CipherException {
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initUi();
        setGasPriceText(10);
        setGasLimit(minimumGasLimit);
        initWeb3j();
    }

    private void initUi() {
        progressBar = (ProgressBar) findViewById(R.id.progressbar);
        progressBar2 = (ProgressBar) findViewById(R.id.progressBar2);
        comprarTextView = (TextView) findViewById(R.id.ComprarText);
        botonNumPakoins = (Button) findViewById(R.id.botonNPakoins);
        nVales = (EditText) findViewById(R.id.NValesCanjear);
        botonNumPakoins.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                getNumPakoins();
            }
        });
        gasPriceSeekBar = (SeekBar) findViewById(R.id.gas_price_seek_bar);
        gasPriceSeekBar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
            @Override
            public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
                setGasPriceText(progress);
            }

            @Override
            public void onStartTrackingTouch(SeekBar seekBar) {
            }

            @Override
            public void onStopTrackingTouch(SeekBar seekBar) {
            }
        });
        SeekBar gasLimitSeekBar = (SeekBar) findViewById(R.id.gas_limit_seek_bar);
        gasLimitSeekBar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
            @Override
            public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
                setGasLimit(progress + minimumGasLimit);
            }

            @Override
            public void onStartTrackingTouch(SeekBar seekBar) {
            }

            @Override
            public void onStopTrackingTouch(SeekBar seekBar) {
            }
        });
        gasLimitTextView = (TextView) findViewById(R.id.gas_limit_text_view);
        gasPriceTextView = (TextView) findViewById(R.id.gas_price_text_view);
    }


    private void writeGreetingToContract() {
        progressBar.setVisibility(View.VISIBLE);
        WriteTask writeTask = new WriteTask();
        writeTask.execute(editText.getText().toString());
    }

    private void  getNumPakoins(){
        try {
            progressBar.setVisibility(View.VISIBLE);
            numPakoins numeroPakoins = new numPakoins();
            numeroPakoins.execute();
        } catch (Exception e) {
            Log.d("wat", "getDiezBonosexception = " + e.getMessage());
        }
    }

    private void initWeb3j() {
        InitWeb3JTask task = new InitWeb3JTask();
        task.execute(rinkebyUrl);
    }

    public void setGasPriceText(int gasPrice) {
        String formattedString = getString(R.string.gas_price, String.valueOf(gasPrice));
        gasPriceTextView.setText(formattedString);
    }

    private BigInteger getGasPrice() {
        int gasPriceGwei = gasPriceSeekBar.getProgress();
        BigInteger gasPriceWei = BigInteger.valueOf(gasPriceGwei + 1000000000L);
        Log.d("wat", "getGasPrice: " + String.valueOf(gasPriceGwei));
        return gasPriceWei;
    }

    public void setGasLimit(int gasLimit) {
        String gl = String.valueOf(gasLimit);
        this.gasLimit = new BigInteger(gl);
        gasLimitTextView.setText(getString(R.string.gas_limit, gl));
    }

    private class numPakoins extends AsyncTask<String, String, String> {
        @Override
        protected String doInBackground(String... params) {
            String result;
            try {
                Pakoin pakoin = Pakoin.load(greeterContractAddressRinkeby, web3j, credentials, getGasPrice(), gasLimit);
                BigInteger balance = pakoin.nPakoins(publicKeyUser).send();
                result = balance.toString();
                //result = transactionReceipt.getBlockHash();
            } catch (Exception e) {
                result = "Error reading the smart contract. Error: " + e.getMessage();
            }

            return result;
        }

        @Override
        protected void onPostExecute(String result) {
            super.onPostExecute(result);
            progressBar.setVisibility(View.INVISIBLE);
            comprarTextView.setText(result);
        }
    }


    private class InitWeb3JTask extends AsyncTask<String, String, String> {
        @Override
        protected String doInBackground(String... params) {
            String url = params[0];
            HttpService infuraHttpService;
            String result = "Success initializing web3j/infura";
            try {
                infuraHttpService = new HttpService(url);
                web3j = Web3j.build(new HttpService(url));
            } catch (Exception wtf) {
                String exception = wtf.toString();
                Log.d("wat", "Error initializing web3j/infura. Error: " + exception);
            }

            return result;
        }

        @Override
        protected void onPostExecute(String result) {
            super.onPostExecute(result);
            Toast.makeText(MainActivity.this, result, Toast.LENGTH_LONG).show();
        }
    }