C#

namespace ClassesnWorkshops
{
    public partial class _Default : System.Web.UI.Page
    {
        #region Web Form Designer generated code
        
        protected System.Web.UI.WebControls.RadioButton rdoProduct;
        protected System.Web.UI.WebControls.Image Image1;

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            this.cmdSearch.Click += new System.EventHandler(this.cmdSearch_Click);
            this.Load += new System.EventHandler(this.Page_Load);
        }

        #endregion


        #region "Local Variables and Declarations"

        private const bool blnAllowPaging = true;
        private const int iPageSize = 5;

        //private static 
        PagedDataSource pgdProducts = new PagedDataSource();

        #endregion

                

        #region Page_Load
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {                
                //Hide results controls
                rptClasses.Visible = false;
                lblMessage.Text = "";
                //set previous link, next link and page# labels to invisible.
                lblCurrentPage.Visible = true;
                lnkNext.Visible = true;
                lnkPrevious.Visible = true;

                //Populate within values (real app would pull from DB)
                cboWithin.Items.Add("Only this zip");
                cboWithin.Items[cboWithin.Items.Count - 1].Value = "0";
                cboWithin.Items.Add("5 miles");
                cboWithin.Items[cboWithin.Items.Count - 1].Value = "5";
                cboWithin.Items.Add("10 miles");
                cboWithin.Items[cboWithin.Items.Count - 1].Value = "10";
                cboWithin.Items.Add("25 miles");
                cboWithin.Items[cboWithin.Items.Count - 1].Value = "25";
                cboWithin.Items.Add("50 miles");
                cboWithin.Items[cboWithin.Items.Count - 1].Value = "50";
                cboWithin.Items.Add("100 miles");
                cboWithin.Items[cboWithin.Items.Count - 1].Value = "100";
                cboWithin.Items.Add("200 miles");
                cboWithin.Items[cboWithin.Items.Count - 1].Value = "200";
                cboWithin.Items.Add("No limit");
                cboWithin.Items[cboWithin.Items.Count - 1].Value = "999";
            }                
        }
        #endregion



        #region "Footer LinkButtons"

        protected void lnkFirst_Click(object sender, EventArgs e)
        {
            //Set viewstatevariable to the next page.
            NowViewing = 0;

            //Reload Control
            BindRepeater();
        }

        protected void lnkPrev_Click(object sender, EventArgs e)
        {
            //Set viewstatevariable to the next page.
            //CurrentPage -= 1;
            NowViewing--;

            //Reload Control
            BindRepeater();
        }

        protected void lnkNext_Click(object sender, EventArgs e)
        {            
            //Set viewstatevariable to the next page
            //CurrentPage += 1;
            NowViewing++;
            //Reload control
            BindRepeater();
        }

        protected void lnkLast_Click(object sender, EventArgs e)
        {
            //Set viewstatevariable to the previous page
            NowViewing = pgdProducts.PageCount - 1;

            //Reload control
            BindRepeater();
        }

        #endregion


        #region "Footer DropDownList"

        protected void ddlpageNumbers_SelectedIndexChanged(object sender, EventArgs e)
        {
            //pgdProducts.PageSize = int.Parse(ddlpageNumbers.SelectedValue);
            NowViewing = ddlpageNumbers.SelectedIndex;
            BindRepeater();
        }

        #endregion


        #region "Custom Functions"

        private void BindRepeater()
        {
            pgdProducts.AllowPaging = blnAllowPaging;
            pgdProducts.PageSize = iPageSize;
            pgdProducts.DataSource = GetProductsDataView();

            pgdProducts.CurrentPageIndex = NowViewing;
            //lblTotalPages.Text = pgdProducts.PageCount.ToString();
            FillPagesDropDownList(pgdProducts.PageCount);
            //tell what page number the user is on and how many pages of results there are total.
            //lblCurrentPageBottom.Text = "Showing Page " + CurPage.ToString() + " of " + pagesCount + " ";
            lblCurrentPage.Text = "Now Showing Page " + (NowViewing + 1).ToString() + " of " + pgdProducts.PageCount.ToString();
            lblItemsPerPage.Text = (pgdProducts.Count).ToString() + " items per page.";
            //lblTotalItemCount.Text = (objPds.Count * objPds.PageCount).ToString() + " total number of items that your search returned.";
            lblTotalItemCount.Text = (pgdProducts.DataSourceCount).ToString() + " total number of items that your search returned.";

            lnkFirst.Enabled = !pgdProducts.IsFirstPage;
            lnkPrevious.Enabled = !pgdProducts.IsFirstPage;
            lnkNext.Enabled = !pgdProducts.IsLastPage;
            lnkLast.Enabled = !pgdProducts.IsLastPage;
            ddlpageNumbers.SelectedIndex = pgdProducts.CurrentPageIndex;

            rptClasses.DataSource = pgdProducts;
            rptClasses.DataBind();
        }

        private DataView GetProductsDataView()
        {
            DataTable dtProductsTemp = new DataTable();
            using (SqlConnection conn = new SqlConnection(ConfigurationManager.AppSettings["ConnStr"]))
            {                
                using (SqlDataAdapter adapProducts = new SqlDataAdapter("cmd", conn))
                {
                    SqlParameter[] arParms = new SqlParameter[3];
                    arParms[0] = new SqlParameter("@Zipcode", SqlDbType.Char);
                    arParms[0].Value = txtZipCode.Text;
                    arParms[1] = new SqlParameter("@Miles", SqlDbType.Decimal);
                    arParms[1].Value = Int16.Parse(cboWithin.SelectedItem.Value);
                    arParms[2] = new SqlParameter("@Subject", SqlDbType.Char);
                    arParms[2].Value = txtSearchWords.Text;

                    DataSet ds = new DataSet();

                    ds = SqlHelper.ExecuteDataset(ConfigurationManager.ConnectionStrings["ClassesnWorkshops.Properties.Settings.ConnectionString"].ConnectionString,
                        CommandType.StoredProcedure, "spHouses_GetNearZipcode", arParms);

                    dtProductsTemp = ds.Tables[0];
                }
            }
            return dtProductsTemp.DefaultView;            
        }


        private void FillPagesDropDownList(int iTotalPages)
        {
            ddlpageNumbers.Items.Clear();
            for (int i = 1; i <= iTotalPages; i++)
            {
                ddlpageNumbers.Items.Add(new ListItem(i.ToString(), i.ToString()));
            }
        }

        #endregion


        #region "Properties"
        
        //int thisPageNumber;
        private int NowViewing
        {
            get
            {
                object obj = ViewState["_NowViewing"];
                if (obj == null)
                    return 0;
                else
                    return (int)obj;
                //return this.thisPageNumber;
            }
            set
            {
                this.ViewState["_NowViewing"] = value;
                //this.thisPageNumber = value;
            }
        }

        #endregion


        #region cmdSearch_Click
        protected void cmdSearch_Click(object sender, EventArgs e)
        {
            DataTable objDT = null;
			try
			{
				//Query the database for classes
                objDT = GetProductsDataView().ToTable();                
                BindRepeater();

				//Any classes found?
				if (objDT.Rows.Count == 0)
				{
					//None found - hide repeater, show message
					rptClasses.Visible = false;
					lblMessage.Text = "No results found";
				}
				else
				{
					// //Houses found - show repeater, hide message					
					rptClasses.Visible = true;
					lblMessage.Text = "";                    
                }
			}
			catch (Exception ex)
			{
				//Add your own error handling here
				lblMessage.Text = "Error: " + ex.ToString();
			}
			finally
            {			
				//Release memory
				objDT = null;
            }           
        }
        #endregion
                       
  } 
}

I have a  C# and Silverlight project with two combo boxes (CB1, CB2)
-CB1 is initialized and populated with few items.
(e.g. car, plane, motorcycle)
-User selects an item (e.g. car) and CB2 gets automatically populated
(e.g. model, year, color) via the cb1_SelectionChanged()
-user selects the item from CB2  (e.g. model) and a datagid is populated accordingly.

Problem:
Once I selected item from CB2 and datagrid is populated correctly, attempting to select a different item from CB1 generates a "Object reference not set to an instance of an object"
To go around just select a different item then back again to the item and that woks but is really annoying. :) 
Any ideas will be greatly appreciated