REST POST: Class is not filled

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C#

ASP.NET

XML

REST

I'm writing a ASP.NET C# REST service which GET's and POST's data. The GET works fine, I've choosen the XML-format to retrieve the info. The POST gives me "System.NullReferenceException: Object reference not set to an instance of an object." What is the reason why the Class "Relatie" doesn't get filled?

XML
    <ArrayOfRelatie xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.datacontract.org/2004/07/Dekra.Synergy.WS.Models">
        <Relatie>
            <BedrijfAdres />
            <BedrijfEmailadres />
            <BedrijfNaam>Generali France</BedrijfNaam>
            <BedrijfPlaats />
            <BedrijfPostcode />
            <BedrijfTelefoon />
            <Classificatie>PRI</Classificatie>
            <ContactAchternaam>--</ContactAchternaam>
            <ContactEmailadres />
            <ContactInitialen />
            <ContactTelefoon />
            <ContactTussenvoegsel />
            <ContactVoornaam />
            <Id>400146</Id>
            <Referentie />
            <Type>C</Type>
            <VoorkeurCC />
        </Relatie>
    </ArrayOfRelatie>

CLICK EVENT
    WebClient _webClient = new WebClient();
            _webClient.Credentials = new NetworkCredential("....", "....");
            _webClient.Headers.Add("Content-type", "application/xml");
            _webClient.Headers.Add("Accept", "application/xml");

            Uri uriGet = new Uri("https://claimsotap.dekra.com/Bizzoscope WS/api/Relatie?code=400146");
        string URL = "https://claimsotap.dekra.com/Bizzoscope WS/api/Relatie";
        string Response = _webClient.DownloadString(uriGet);

        byte[] bytes = Encoding.UTF8.GetBytes(Response);

        HttpWebRequest request = CreateWebRequest(URL, bytes.Length);

        using (var requestStream = request.GetRequestStream())
        {
            requestStream.Write(bytes, 0, bytes.Length);
        }

        using (var response = (HttpWebResponse)request.GetResponse())
        {
            if (response.StatusCode != HttpStatusCode.Created && response.StatusCode != HttpStatusCode.OK)
            {
                string message = String.Format("POST failed. Received HTTP {0}", response.StatusCode);
                throw new ApplicationException(message);
            }
        }
public HttpResponseMessage Post(Relatie relatie)
        {
            logger.Logger.Info("SaveRelatie");
            this.relatieRepository.SaveRelatie(relatie);

            var response = Request.CreateResponse<Relatie>(System.Net.HttpStatusCode.Created, relatie);

            return response;
        }

    public bool SaveRelatie(Relatie relatie)
    {
        try
        {
            Functions func = new Functions();
            Guid generatedGuid = new Guid();
            string caseValidationTable = "_CONN_TB_Csnob1799traffic";
            logger.Logger.Info("RelatieRepository");
            func.CreateRelatie(relatie, generatedGuid, caseValidationTable);
            logger.Logger.Info("RELATIE " + relatie.Id + " AANGEMAAKT");
            return true;
        }
        catch (Exception ex)
        {
            logger.Logger.ErrorException("Fout tijdens het aanmaken van de relatie.", ex);
            return false;
        }
    }

    public Guid CreateRelatie(Relatie relationDetails, Guid generatedGuid, string projectNr)
    {
        logger.Logger.Info("Functions");
        try
        {
            if (relationDetails != null) // HERE IT GOES WRONG
            {
                logger.Logger.Info("relDet");
                 ......