Hi, I am getting this error at runtime: object reference not set to an instance of object

my question is that am i using stringbuilder array correctly here. Because I am new in C#. and i think its the problem with my stringbuilder array. Below is the code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Services;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Text;
using System.Web.Script.Serialization;
using System.Web.Script.Services;
using System.Collections;


public partial class Testing : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }


    [WebMethod]
    public static string SendMessage()
    {
        try
        {
            al2c00.ldap ws = new al2c00.ldap();
            Hashtable htPeople = new Hashtable();

            //DataTable dt = ws.GetEmployeeDetailsBy_NTID("650FA25C-9561-430B-B757-835D043EA5E5", "john");

            StringBuilder[] empDetails = new StringBuilder[100];


            string num = "ambreen";
            empDetails[0].Append("amby");
            num = empDetails[0].ToString();

            htPeople.Add("bellempposreport", num);


            JavaScriptSerializer jss = new JavaScriptSerializer();

            string output = jss.Serialize(htPeople);
            return output;
        }
        catch(Exception ex)
        {
            return ex.Message + "-" + ex.StackTrace;
        }
    }
}

please reply me what i am doing wrong here.