java .lang.IllegalStateException: ожидалось BEGIN_OBJECT, но было STRING в строке 2 пути 1 столбца $ kotlin

Question

Я использую Node.js, express. js и mongodb в качестве бэкэнда. И я использую kotlin в android для внешнего интерфейса. Я строю приложение сканера штрих-кода. Штрих-код содержит электронную почту пользователя. И электронная почта передается на сервер и проверяет (изменить столбец БД) пользователя. LogCat показывает ошибку, как в заголовке выше. Я протестировал js, и проблем нет. Я думаю, что проблема исходит от GSON. Но я не знаю, как ее решить.

MainActivity.kt

class MainActivity : AppCompatActivity() {

    private  lateinit var retrofit: Retrofit
    private lateinit var retrofitInterface: RetrofitInterface
    private val BASE_URL = "URL"

    private lateinit var svBarcode: SurfaceView
    private lateinit var tvBarcode: TextView

    private lateinit var detector: BarcodeDetector
    private lateinit var cameraSource: CameraSource

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        var gson = GsonBuilder()
            .setLenient()
            .create()

        retrofit = Retrofit.Builder().baseUrl(BASE_URL).addConverterFactory(GsonConverterFactory.create(gson)).build()
        retrofitInterface = retrofit.create(RetrofitInterface::class.java)
        svBarcode = findViewById(R.id.sv_barcode)
        tvBarcode = findViewById(R.id.tv_barcode)

        detector = BarcodeDetector.Builder(this).setBarcodeFormats(ALL_FORMATS).build()

        detector.setProcessor(object :Detector.Processor<Barcode> {
            override fun release() {}

            override fun receiveDetections(detections: Detector.Detections<Barcode>) {
                val barcode = detections.detectedItems
                if (barcode!!.size() > 0){
                    var map:HashMap<String, String> = HashMap()

                    map.put("email",barcode.valueAt(0).displayValue.toString())

                    var call = retrofitInterface.executeCheckUser(map)

                    call.enqueue(object: Callback<UserResult> {
                        override fun onFailure(call: Call<UserResult>, t: Throwable) {
                            Log.d("Error Callback", t.message.toString())
                        }

                        override fun onResponse(
                            call: Call<UserResult>,
                            response: Response<UserResult>
                        ) {
                            if(response.code() == 200) {
                                Toast.makeText(this@MainActivity, "Participant attendance confirmed", Toast.LENGTH_SHORT).show()
                            } else if (response.code() == 100) {
                                Toast.makeText(this@MainActivity, "Already Confirmed", Toast.LENGTH_SHORT).show()
                            }
                            if (response.code() == 250) {
                                val result = response.body()
                                if (result != null) {
                                    tvBarcode.text ="Nama: " + result.name + "\nEmail: " + result.email + "\nInstitusi" + result.institution + "\nProgram Studi" + result.program
                                }
                            }
                            if(response.code() == 404) {
                                Toast.makeText(this@MainActivity, "Participant not found", Toast.LENGTH_SHORT).show()
                            }
                        }
                    })
                }
            }
        })

        cameraSource = CameraSource.Builder(this,detector).setRequestedPreviewSize(1080,1080)
            .setRequestedFps(30f).setAutoFocusEnabled(true).build()
        svBarcode.holder.addCallback(object :SurfaceHolder.Callback2{
            override fun surfaceRedrawNeeded(p0: SurfaceHolder?) {}

            override fun surfaceChanged(p0: SurfaceHolder?, p1: Int, p2: Int, p3: Int) {}

            override fun surfaceDestroyed(p0: SurfaceHolder?) {
                cameraSource.stop()
            }

            override fun surfaceCreated(holder: SurfaceHolder?) {
                if (ContextCompat.checkSelfPermission(this@MainActivity,
                        android.Manifest.permission.CAMERA) ==
                    PackageManager.PERMISSION_GRANTED)
                    cameraSource.start(holder)
                else ActivityCompat.requestPermissions(this@MainActivity,
                    arrayOf(android.Manifest.permission.CAMERA),123)
            }
        })
    }

    @SuppressLint("MissingPermission")
    override fun onRequestPermissionsResult(requestCode: Int, permissions: Array<out String>, grantResults: IntArray) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults)
        if (requestCode == 123){
            if (grantResults.isNotEmpty() && grantResults[0] == PackageManager.PERMISSION_GRANTED)
                cameraSource.start(svBarcode.holder)
            else Toast.makeText(this,"Camera permission is needed",Toast.LENGTH_SHORT)
                .show()
        }
    }

    override fun onDestroy() {
        super.onDestroy()
        detector.release()
        cameraSource.stop()
        cameraSource.release()
    }
}

RetrofitInterface. java

public interface RetrofitInterface {

    @POST("/")
    Call<UserResult> executeCheckUser(@Body HashMap<String, String> map);
}

UserResult. java

public class UserResult{

    public String getName() {
        return name;
    }

    public String getEmail() {
        return email;
    }

    public String getInstitution() {
        return institution;
    }

    public String getProgram() {
        return program;
    }

    private String name;
    private String email;
    private String institution;
    private String program;
}

приложение. js

const express = require('express')
const app = express()
const mongoClient = require('mongodb').MongoClient

const url = dbURL

app.use(express.json())

mongoClient.connect(url, (err, db) => {

    if (err) {

        console.log("Error while connectiong mongo client")

    } else {

        console.log("Connection success")

        const myDb = db.db('myDb')
        const collection = myDb.collection('myCollection')

        app.post('/', (req, res) => {

            const participant = {
                email: req.body.email
            }

            const query = { email: participant.email }
            const newvalues = { $set: { hasAttended: true } };

            collection.findOne(query, (err, result) => {
                console.log("Result " + result)

                if (result != null) {

                    if (result.hasAttended == true) {

                        console.log("User has attended")
                        res.status(100).send()

                    } else {

                        collection.updateOne(query, newvalues, (err, result) => {
                            console.log("User attendence updated")
                            res.status(200).send()
                        })

                    }

                    const objToSend = {
                        name: result.name,
                        email: result.email,
                        institution: result.institution,
                        program: result.program
                    }

                    res.status(250).send(JSON.stringify(objToSend))
                    console.log(JSON.stringify(objToSend))

                } else {

                    console.log("Can't find user")
                    res.status(404).send()

                }
            })
        })
    }
})