Listbox on an AJAX TAB CONTAINER postback problem

[mimifara]

Good Morning,
I have a problem with my listbox control, and i tried everything i found on the web but maybe i'm still missing something.
I have a listbox that i control with a datatable <the data table is populated through a data layer which is a seperate application> and i bind it to the listbox,
on selecting an item from that listbox, a second dropdownlistbox should be activated.

My problem is that when i do postback to activate the second dropdownlistbox , the first get emptied, i dont know whats the problem.

here is my code.
<source code>

<asp:DropDownList ID="IndexDropDownList" runat="server" AutoPostBack="True" AppendDataBoundItems="True"
                                            Height="18px" Width="301px" 
                                            onselectedindexchanged="IndexDropDownList_SelectedIndexChanged">

on behind page code is

public partial class markit_Index : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            DataTable data = new DataTable();
 
            Getdata();
      }
   }
 
protected void  Getdata ()
    {
        DataSet MyDataset = new DataSet();
 
 
        CurveDao iDao = new CurveDao("", ContextManager.oracleSession);
 
        DataTable data = iDao.GetIndexCompositionData();
        Session["GetIndexCompositionData"]= data;
 
        DataTable table = new DataTable();
        table.Columns.Add("Index Name");
        foreach (DataRow row in data.Select())
        {
            DataRow NewRow = table.NewRow();
            String IndexName = row["Name"].ToString();
            NewRow["Name"] = Name;
                 table.Rows.Add(NewRow);
        }
        IndexDropDownList.DataSource =table;
        IndexDropDownList.DataTextField = "Name";
        IndexDropDownList.DataBind();
 
    }
 
 protected void IndexDropDownList_SelectedIndexChanged(object sender, EventArgs e)
    {
 
 
        IndexNameDropDownList.Enabled = true;
 
     }
 
}

Thanks for the help !!