method error 500 filled in second cascading dropdown list?

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C#

my web service code is...

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
[System.Web.Script.Services.ScriptService()]

public class CasCadingDropDown : System.Web.Services.WebService
{
    //private DataSet ds = new DataSet();
    //private DataTable dt = new DataTable();
    public CasCadingDropDown()
    {

        //Uncomment the following line if using designed components 
        //InitializeComponent(); 
    }
    
    [WebMethod]
    [System.Web.Script.Services.ScriptMethod]
    public CascadingDropDownNameValue[] GetStates(string knownCategoryValues, string category)
    {
        StringDictionary ck = CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);
        string SomeOtherThing = "";
        if (!ck.ContainsKey("SomeOtherThing") || ck["SomeOtherThing"] == null)
        {
            SomeOtherThing = ck["SomeOtherThing"];
            
        }
        else
        {
            return null;
        }
     using(NpgsqlConnection conn=new NpgsqlConnection(ConfigurationSettings.AppSettings["ConnectionString"].ToString()))
    {
      using(NpgsqlCommand cmd=new NpgsqlCommand("getstates",conn))
      {
        cmd.CommandType=CommandType.StoredProcedure;
        
        conn.Open();
        List<cascadingdropdownnamevalue> states = new List<cascadingdropdownnamevalue>();
            using (NpgsqlDataReader reader = cmd.ExecuteReader())
                {
                    while (reader.Read())
                    {
                        string StateId = (string)reader[0].ToString();
                        string StateName = (string)reader[1].ToString();
                        if(!string.IsNullOrEmpty(SomeOtherThing))
                        {
                            if(SomeOtherThing==StateId)
                        states.Add(new CascadingDropDownNameValue(StateName, Convert.ToString(StateId),true));
                        }
                        else
                        {
                             states.Add(new CascadingDropDownNameValue(StateName, Convert.ToString(StateId)));
                        }
                    }
                }
          return states.ToArray();
        
      }
        
     }
    }
 [WebMethod]
    [System.Web.Script.Services.ScriptMethod]
    public CascadingDropDownNameValue[] GetDistricts(string knownCategoryValues, string category)
    {

        StringDictionary kv = CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);
        int StateId;
        if (!kv.ContainsKey("States") || !Int32.TryParse(kv["States"], out StateId))
        {
            return null;
        }
        using (NpgsqlConnection conn = new NpgsqlConnection(ConfigurationSettings.AppSettings["ConnectionString"].ToString()))
        {
            using (NpgsqlCommand cmd = new NpgsqlCommand("getdistricts", conn))
            {
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.Add(new NpgsqlParameter("_stateid", StateId));
                // cmd.Parameters.Add(new NpgsqlParameter("_stateid",DbType.Int32,int.MaxValue,"stateid",ParameterDirection.Input,false,Byte.MaxValue,byte.MaxValue,DataRowVersion.Default,Formats.ConvertToInt(StateId)));
                //cmd.Parameters.Add("_stateid", StateId);
                conn.Open();
                List<cascadingdropdownnamevalue> districts = new List<cascadingdropdownnamevalue>();
                using (NpgsqlDataReader rdr = cmd.ExecuteReader())
                {
                    while (rdr.Read())
                    {
                        string districtid = (string)rdr[0].ToString();
                        string districtName = (string)rdr[1].ToString();
                        districts.Add(new CascadingDropDownNameValue(districtName, districtid));
                    }
                }
                return districts.ToArray();
            }

        }
    }

states cascading dropdown works good. when i select state the district drop down filled with method error 500..i put the break point and see the execution flow the cursor goes to end after using (NpgsqlDataReader rdr = cmd.ExecuteReader())