Android Question getstring unknown

Sub Process_Globals
    Dim hc As OkHttpClient
End Sub

Sub Activity_Create(FirstTime As Boolean)
        If FirstTime Then
        hc.Initialize("hc")
    End If
        ......
        ......
End Sub

Sub pgcertificatoBTInvia_Click
            'Add files
            Dim files As List
            files.Initialize
            Dim FD As FileData
            FD.Initialize
            FD.Dir = File.DirDefaultExternal
            FD.FileName = nomefilecertificato
            FD.KeyName = "upfilecert"
            FD.ContentType = "application/octet-stream"
            files.Add(FD)

            'Add name / values pairs (parameters)
            Dim NV As Map
            NV.Initialize
            NV.Put("matricola", Main.WFAtletaId)
            NV.Put("id_associazione ", Main.WFStrutturacod)
            NV.Put("nomefile", nomefilecertificato)
            Dim req As HttpRequest
            req = MultipartPost.CreatePostRequest("http://www.example.com/1.php", NV, files)
            hc.Execute(req, 1)
        Else
            messaggio=ML.ML_pgcertificato_msgbox_scegli
            Msgbox2(messaggio, "webfitness","OK","","", LoadBitmap(File.DirAssets, "wflogo.png"))
        End If
end sub

Sub hc_ResponseError (Response As OkHttpResponse, Reason As String, StatusCode As Int, TaskId As Int)
   Log("error: " & Response & " " & StatusCode)
   If Response <> Null Then
       Log(Response.GetString("UTF8"))
       Response.Release
   End If
End Sub
Sub hc_ResponseSuccess (Response As okHttpResponse, TaskId As Int)
   Msgbox(Response.GetString("UTF8"), "")
   Response.Release
End Sub