请大家帮忙看看着段TreeView连接oracle的代码，谢谢了（急）

<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="TreeViewDynamic.aspx.cs" Inherits="_Default" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>无标题页</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:Label ID="labelStatus" runat="server" Style="z-index: 100; left: 46px; position: absolute;
            top: 49px" Text="labelStatus"></asp:Label>
        <asp:TreeView ID="TreeView1" runat="server" MaxDataBindDepth="2" OnTreeNodePopulate="TreeView1_TreeNodePopulate"
            Style="z-index: 102; left: 42px; position: absolute; top: 100px">
            <Nodes>
                <asp:TreeNode PopulateOnDemand="True" Text="产品列表" Value="产品列表"></asp:TreeNode>
            </Nodes>
        </asp:TreeView>
    
    </div>
        <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
            ProviderName="<%$ ConnectionStrings:ConnectionString.ProviderName %>" SelectCommand='SELECT "CATEGORYID", "CATEGORYNAME" FROM "CATEGORIES"'>
        </asp:SqlDataSource>
    </form>
</body>
</html>using System;
using System.Data;
using System.Data.OracleClient;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {    }
   
        protected void TreeView1_TreeNodePopulate(
            object sender, TreeNodeEventArgs e)
        {
            if (e.Node.ChildNodes.Count == 0)
            {
                switch (e.Node.Depth)
                {
                    case 0:
                        PopulateCategories(e.Node);
                        break;
                    case 1:
                        PopulateProducts(e.Node);
                        break;
                }
            }
        }    void PopulateCategories(TreeNode node)
    {
        OracleCommand sqlQuery = new OracleCommand(
            "Select CategoryName, CategoryID From Categories");
        DataSet resultSet;
        resultSet = RunQuery(sqlQuery);
        if (resultSet.Tables.Count > 0)
        {
            foreach (DataRow row in resultSet.Tables[0].Rows)
            {
                TreeNode NewNode = new
                    TreeNode(row["CategoryName"].ToString(),
                    row["CategoryID"].ToString());
                NewNode.PopulateOnDemand = true;
                NewNode.SelectAction = TreeNodeSelectAction.Expand;
                node.ChildNodes.Add(NewNode);
            }
        }
    }    void PopulateProducts(TreeNode node)
    {
        OracleCommand sqlQuery = new OracleCommand();
        sqlQuery.CommandText = "Select ProductName From Products " +
            " Where CategoryID = @categoryid";
        sqlQuery.Parameters.Add("@categoryid", SqlDbType.Int).Value =
            node.Value;
        DataSet ResultSet = RunQuery(sqlQuery);
        if (ResultSet.Tables.Count > 0)
        {
            foreach (DataRow row in ResultSet.Tables[0].Rows)
            {
                TreeNode NewNode = new
                    TreeNode(row["ProductName"].ToString());
                NewNode.PopulateOnDemand = false;
                NewNode.SelectAction = TreeNodeSelectAction.None;
                node.ChildNodes.Add(NewNode);
            }
        }
    }
    private DataSet RunQuery(OracleCommand sqlQuery)
    {
        string connectionString = ConfigurationManager.ConnectionStrings["NorthwindConnectionString"].ConnectionString;
        OracleConnection DBConnection =
            new OracleConnection(connectionString);
        OracleDataAdapter dbAdapter = new OracleDataAdapter();
        dbAdapter.SelectCommand = sqlQuery;
        sqlQuery.Connection = DBConnection;
        DataSet resultsDataSet = new DataSet();
        try
        {
            dbAdapter.Fill(resultsDataSet);
        }
        catch
        {
            labelStatus.Text = "Unable to connect to SQL Server.";
        }
        return resultsDataSet;
    }
}
我运行的时候，string connectionString = ConfigurationManager.ConnectionStrings["NorthwindConnectionString"].ConnectionString;
这段代码报错，提示未将对象引用设置到对象的实例。请问这是什么原因？我这些代码都是从MSDN里面复制出来的。只是把SQL改成oracle数据库，谁能帮我看看这代码，哪错了，谢谢了。