Update a record in PODIO via API.NET

Hello!
I can not update an existing item in PODIO. Yes, a simple update of a given.
I've read the documentation and consulted the forum. But not works.
Someone managed to update a record in PODIO via API.NET? I tried two ways and both led to the following error:

I consulted this forum, but it not worked.
https://help.podio.com/hc/communities/public/questions/202868606-Modifying-items-C-client-library

First try!
Error: "Object reference not set to an instance of an object."}

public void Update(Cobranca cobranca)
{
//Connect
var _podio = new PodioAPI.Podio("integracao-cobranca-ac", chaveAreaTrabalho);
_podio.AuthenticateWithApp(idAplicativo, codigoAplicativo);

        var opcaoFiltro = new FilterOptions();
        var filtro = new Dictionary<string, object>();
        if (cobranca.NumeroTitulo != null)
        {
            filtro.Add("titulo", cobranca.NumeroTitulo); 
        }
        opcaoFiltro.Filters = filtro;
        var filteredContent = _podio.ItemService.FilterItems(idAplicativo, opcaoFiltro);
        if (filteredContent.Items.Any())
        {
            //loop items for update
            foreach (var item in filteredContent.Items)
            {
                Item itemUpdate = new Item();
                itemUpdate .ItemId = item.ItemId;

                var statusBackoffice = itemUpdate.Field<CategoryItemField>("status-backoffice");
                statusBackoffice.OptionId = (int)eStatusBackOffice.NegociadoBack;

                //Update item podio
                _Podio.ItemService.UpdateItem(itemUpdate);
    }
         }

}

Second attempt!
Error: "Object reference not set to an instance of an object."}
public void Update(Cobranca cobranca)
{
//Connect
var _podio = new PodioAPI.Podio("integracao-cobranca-ac", chaveAreaTrabalho);
_podio.AuthenticateWithApp(idAplicativo, codigoAplicativo);

        var opcaoFiltro = new FilterOptions();
        var filtro = new Dictionary<string, object>();
        if (cobranca.NumeroTitulo != null)
        {
            filtro.Add("titulo", cobranca.NumeroTitulo); 
        }
        opcaoFiltro.Filters = filtro;
        var filteredContent = _podio.ItemService.FilterItems(idAplicativo, opcaoFiltro);
        if (filteredContent.Items.Any())
        {
            //loop items for update
            foreach (var item in filteredContent.Items)
            {
                var statusBackoffice = item.Field<CategoryItemField>("status-backoffice");
                statusBackoffice.OptionId = (int)eStatusBackOffice.NegociadoBack;

                //Update item podio
                _Podio.ItemService.UpdateItem(item);
            }
        }

}