How to display the selected node from the treeview in textbox.

Hi im having treeview with checkbox and 1 submit button it is taking values from database.

now if i check the checkbox and click the submit button it should display the selected node in the textbox.....

here is my treeview code

    public void Load_tree()         {             DataSet PrSet = PDataset("SELECT * FROM tbl_saleschannelhierarchyconfig");             TreeView1.Nodes.Clear();             foreach (DataRow dr in PrSet.Tables[0].Rows)             {                 if ((int)dr["ParentID"] == 0)                 {                     TreeNode tnParent = new TreeNode();                     tnParent.Text = dr["Parent"].ToString();                     string value = dr["ParentID"].ToString();                     //tnParent.Text = dr["SalesChannelName"].ToString(); - if u use this   instead of the previous two lines, then "kmart" will be the parent. else "root" will be the parent.                     //string value = dr["SalesChannelConfigID"].ToString();                     tnParent.Expand();                     TreeView1.Nodes.Add(tnParent);                     FillChild(tnParent, value);                 }             }         }               public int FillChild(TreeNode parent, string SalesChannelConfigID)         {             DataSet ds = PDataset("SELECT * FROM tbl_saleschannelhierarchyconfig WHERE   ParentID =" + SalesChannelConfigID);             if (ds.Tables[0].Rows.Count > 0)             {                 foreach (DataRow dr in ds.Tables[0].Rows)                 {                     TreeNode child = new TreeNode();                     child.Text = dr["SalesChannelName"].ToString().Trim();                     string temp = dr["SalesChannelConfigID"].ToString();                     child.Collapse();                     parent.ChildNodes.Add(child);                     FillChild(child, temp);                 }                 return 0;             }             else             {                 return 0;             }

and button click code

       protected void Button1_Click(object sender, EventArgs e)         {

            txtRetailCustomerGroup.Text = TreeView1.SelectedNode.Parent.ToString();         }

it is showing this exception "Object reference not set to an instance of an object."

can any1 help me??