ASP.NET

.ascx

 public string CountryName
        {
            get { return lblCountry.Text; }
            set 
            {
                if (!(value == ""))
                    lblCountry.Text = value;
                else
                    lblCountry.Text = "All Country";
            }
        }

        public string AdType
        {
            get { return lblAdType.Text; }
            set 
            {
                if (!(value == ""))
                    lblAdType.Text = value;
                else
                    lblAdType.Text = "All Ads";
            }
        }
.aspx
------
   public string AdType
        {
            get { return _AdType; }
            set { _AdType = value; }
        }
if (!Page.IsPostBack)
{
  if (!(Request.QueryString["q"] == null))
                {
                    Session["AdType"] = Request.QueryString["q"];
                }
 if (Session["AdType"] != null)
                {
                    AdType = Session["AdType"].ToString();
                    ddAdType.SelectedIndex = ddAdType.Items.IndexOf(ddAdType.Items.FindByText(AdType));
                }

  this.Left.AdType = ddAdType.SelectedItem.Text; <-- Left is a ascx here. and null ref exception is thrown at this line.
                this.Left.CountryName = CountryName;
            }