How to get real document file (PDF) and upload it using Retrofit in Kotlin

Question

I am trying to get a document file (PDF) from Android storage, then I want to upload PDF files where I get earlier to the server using Retrofit. I just tried to get and upload it, but the files are not uploaded. It says my files are null. I used Kotlin and here's my code

My Activity

class SignatureUploadFileActivity : AppCompatActivity() {
    private val vm: SignatureViewModel by viewModel()
    val CREATE_FILE = 1
    private lateinit var file: File
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_signature_upload_file)
        initAction()
        initObserver()
    }

    private fun initAction() {
        ll_upload.setOnClickListener {
            getpdf()
        }
        btn_signature_upload.setOnClickListener {
            uploadfile()
        }
    }

    private fun uploadfile() {
        var body: MultipartBody.Part?
        val reqFile = RequestBody.create("application/pdf".toMediaTypeOrNull(), file)
        body = MultipartBody.Part.createFormData("profile_picture", file.getName(), reqFile)
        vm.signex(body)
    }

    private fun initObserver() {
        vm.signEx.observe(this) {
            if (it != null) {
                Log.d("Upload File", "Success ${it.message}")
            }
        }
    }

    private fun getpdf() {
        val intent = Intent(Intent.ACTION_GET_CONTENT).apply {
            addCategory(Intent.CATEGORY_OPENABLE)
            type = "application/pdf"
        }
        intent.putExtra(Intent.EXTRA_ALLOW_MULTIPLE, true)
        startActivityForResult(intent, CREATE_FILE)
    }

    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)
        when (requestCode) {
            CREATE_FILE -> if (resultCode == Activity.RESULT_OK) {
                // Set up an Intent to send back to apps that request a file
                if (null != data) {
                    val uri = data.data!!
                    file = File(uri.path)
                }
            }
        }
    }
}

My API Interface

@Multipart
@POST("/dev/upload")
suspend fun signEx(
    @Part file: MultipartBody.Part?
): SignExResponse

My Repository

class SignatureRepository(private val webApi: APIServices) {
    suspend fun signatureex(file:MultipartBody.Part):SignExResponse {
        lateinit var response: SignExResponse
        try {
            response = webApi.signEx(file)
            Log.d("Signature Mandiri", "Succesfull")
        } catch (e: Exception) {
            Log.d("Signature Mandiri", "Failed")
        }
        return response
    }
}

Here's my viewmodel

class SignatureViewModel(private val repository: SignatureRepository) : ViewModel() {
    private var _signexresponse = MutableLiveData<SignExResponse>()
    val signEx: LiveData<SignExResponse>
        get() = _signexresponse

    fun signex(file:MultipartBody.Part) {
        viewModelScope.launch {
            try {
                val response = repository.signatureex(file)
                _signexresponse.value = response
            } catch (e: Exception) {

            }
        }
    }
}