Error in treeview while populate from sql

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C#

ASP.NET

Hello

i get this error while trying to populate the treeview from sql database

the error
Object reference not set to an instance of an object

the code

private void PopulateTreeview()
    {

        SqlConnection con = new SqlConnection(connectionString);
        SqlCommand com = new SqlCommand("GLD_CHART_OF_ACCOUNTS_some_select", con);
        com.CommandType = CommandType.StoredProcedure;

        con.Open();
        SqlDataAdapter adbt = new SqlDataAdapter(com);
        DataSet ds = new DataSet();
        com.ExecuteNonQuery();
        adbt.Fill(ds, "GLD_CHART_OF_ACCOUNTS");
        con.Close();
        DataTable dt = ds.Tables[0];
        for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
        {
            string nodeID = ds.Tables[0].Rows[i][0].ToString(); //Get the node code 'Node number'
            string nodeName = ds.Tables[0].Rows[i][1].ToString(); // Get the node name
            string Fullname = nodeID + nodeName; //Concatenate the number + name to get the full node name
            string parent_ID = ds.Tables[0].Rows[i][2].ToString(); //Get the parent ID of the node

            if (parent_ID == "0")
            {
                TreeNode node = new TreeNode();
                node.Text = Fullname;
                TreeView1.Nodes.Add(node);
            }
            else
            {
                //GLD_CHART_OF_ACCOUNTSSelect
                SqlCommand command = new SqlCommand("GLD_CHART_OF_ACCOUNTSSelect", con);
                command.CommandType = CommandType.StoredProcedure;
                SqlDataAdapter adp = new SqlDataAdapter(command);
                DataSet IdSet = new DataSet();
                {
                    con.Open();
                    command.Parameters.Add("@ACCOUNT_CODE", DbType.Int32).Value = Convert.ToInt32(parent_ID);
                }
                command.ExecuteNonQuery();
                adp.Fill(IdSet, "GLD_CHART_OF_ACCOUNTS");
                con.Close();
                string Parent_name = IdSet.Tables[0].Rows[0][0].ToString() + IdSet.Tables[0].Rows[0][1].ToString();
                TreeNode node = new TreeNode();
                node.Text = Fullname;
                TreeView1.FindNode(Parent_name).ChildNodes.Add(node);
            }
        }

    }

when i try to add child node to child
like

node A
sub A
Sub Of sub A
return this error
any help