Error with vectors c # / {"Object reference not set to an instance of an object."}

Friends I'm coming in php and starting to develop in C #. The differences are many ideas that I have when I will apply give errors such as the problem that I have a combobox html that dynamically filled with database data, I thought separates the code which would be the value of the combobox and put as a value in each position from an array up to that, but it does not work. The current version of the following error, does anyone have any idea how I solve?

Give this error:

  

"Object reference not set to an instance of an object."

Code;

using System;
using System.Configuration;
using System.Data;
using System.Net.Http;
using System.Text;
using System.Web;
using Newtonsoft.Json.Linq;

namespace empresa.UI.ProcessEmpresa
{
    /// <summary>
    /// Summary description for AjaxPrazoMedioCliente
    /// </summary>
    public class AjaxPrazoMedioCliente : IHttpHandler
    {
        string[] v;//CRIACAO DO VETOR 
        public int i, j = 0;

        public void ProcessRequest(HttpContext context)
        {
            Grupo = Convert.ToString(context.Request.QueryString["codgrupo"]);
            temmontante = Convert.ToString(context.Request.QueryString["temmontante"]);
            selecionado = Convert.ToString(context.Request.QueryString["selecionado"]);
            temmontante = temmontante.Equals("") ? "0" : temmontante;
            HttpClient client = null;
            string retorno = string.Empty;
            if (client == null)
            {
                client = new HttpClient();
                client.BaseAddress = new Uri(ConfigurationManager.AppSettings["url_servidor"]);
                client.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));
                HttpResponseMessage resultado = client.GetAsync("xyz/Venda/PrazoMedio/" + Grupo + "/" + temmontante).Result;
                retorno = resultado.Content.ReadAsStringAsync().Result;

                if (resultado.StatusCode != System.Net.HttpStatusCode.NotFound)
                {
                    if (retorno != "[]")
                {
                    sb.Append("<select name=\"txtpagpedido\" id=\"txtpagpedido\" onchange=\"HabilitarDescontoPromocional()\" style=\"width: 200px;\" >");
                    sb.Append("<option value=\"0\">Selecione..</option>");
                    JArray usuarioarrray = JArray.Parse(retorno);

                    foreach (JObject obj in usuarioarrray.Children<JObject>())
                    {
                        foreach (JProperty prop in obj.Properties())
                        {
                            if (v[i] != null)//MEU TRECHO PESQUISEI QUE TALVEZ SOLUCIONARIA O ERRO
                                if (Convert.ToString(prop.Value) != v[i])
                                    v[i] = prop.Value.ToString();
                                else
                                    break;

                            //vetor[i] = Convert.ToString(prop.Value.ToString());

                            switch (prop.Name)
                            {
                                case "e4_CODIGO":
                                    if (prop.Value.ToString().Equals(selecionado))
                                        sb.Append("<option value=\"" + Convert.ToString(prop.Value.ToString()) + "\" selected=\"selected\" >");
                                    else
                                        sb.Append("<option value=\"" + Convert.ToString(prop.Value.ToString()) + "\"  >");
                                    break;
                                case "e4_DESCRI":
                                    sb.Append(Convert.ToString(prop.Value.ToString()) + "</option>");
                                    break;
                                default:
                                    break;
                            }
                            i++;
                        }
                    }

                    sb.Append("</select>");

                }

            }

            context.Response.Write(sb.ToString());
        }

        public bool IsReusable
        {
            get
            {
                return false;
            }
        }
    }
}