How can I read the value of a custom characteristic?

I am trying to read the data of a custom characteristic from a device that runs Mynewt.

Until the point that I discover all the services everything works fine.

Here is the part of my code.

@Override
    public void onServicesDiscovered(BluetoothGatt gatt, int status) {
        super.onServicesDiscovered(gatt, status);
        if (status == 0){
            List<BluetoothGattService> services = gatt.getServices();
            peripheralTextView.append("onServicesDiscovered " + services.toString() + "nn");
            readCustomCharacteristic();
        }
    }

    public void readCustomCharacteristic() {
        if (btAdapter == null || mGatt == null) {
            peripheralTextView.append("BluetoothAdapter not initialized n");
            return;
        }
    /*check if the service is available on the device*/
        BluetoothGattService mCustomService = mGatt.getService(UUID.fromString("59462F12-9543-9999-12C8-58B459A2712D"));
        if(mCustomService == null){
            peripheralTextView.append("Custom BLE Service not found n");
            return;
        }
    /*get the read characteristic from the service*/
        BluetoothGattCharacteristic mReadCharacteristic = mCustomService.getCharacteristic(UUID.fromString("5C3A659E-897E-45E1-B016-007107C96DF6"));
        mGatt.readCharacteristic(mReadCharacteristic);

        if(mReadCharacteristic == null){
            peripheralTextView.append("Failed to read characteristic n");
        }
        else {
            peripheralTextView.append("Characteristic " + mGatt.getService(serviceUUID).getCharacteristic(characteristicUUID).toString() + "n");
        }
        return;
    }

    @Override
    public void onCharacteristicRead(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status) {
        peripheralTextView.append("The characteristic UUID is : " + characteristic.getUuid());
        peripheralTextView.append("The value is : " + characteristic.getStringValue(0));
    }

I have also tried other methods for reading the data such as valueOf, getValue etc.

With some of these methods I am getting as an answer null and with some others the whole line is not even print on my screen.

The out put on my mobile screen is this one
Screenshot

I always run my app on my phone: Samsung Galaxy A5 (2016)

Is there any other way/method to read the data from the characteristic or do I miss something?

Thanks in advance