关于asp.net:{“对象引用未设置为对象的实例。“}

{“Object reference not set to an instance of an object.”}

用C_创建一个ASP.NET窗体，我面临着这个错误，我不知道它有什么错误。一切正常，但当我按下"保存"按钮时，会出现以下错误：

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NulllRefrenceException was unhandled BY USER code
{"Object reference not set to an instance of an object."}
Object reference NOT SET TO an instance OF an object.

代码：

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protected void Button8_Click(object sender, EventArgs e)
{
SqlConnection cnn = NEW SqlConnection();
cnn.ConnectionString = System.Configuration.ConfigurationManager.ConnectionStrings["sqlAddSave"].ConnectionString;
cnn.Open();
SqlCommand cmd = NEW SqlCommand();
cmd.CommandText ="select * from DisplayPP";
cmd.Connection = cnn;
SqlDataAdapter da = NEW SqlDataAdapter();
da.SelectCommand = cmd;
DataSet ds = NEW DataSet();
da.Fill(ds," DisplayPP");
SqlCommandBuilder cb = NEW SqlCommandBuilder(da);
DataRow drow = ds.Tables["DisplayPP"].NewRow();
drow["website"] = web.Text;
drow["country"] = DropDownList1.SelectedItem.Text;
drow["contact"] = TextBox144.Text;
drow["cat"] = TextBox145.Text;
drow["traff"] = TextBox146.Text;

more text boxes AS above

ds.Tables["DisplayPP"].Rows.Add(drow);
da.Update(ds," DisplayPP");
string script = @"<script language=""javascript"">
alert('Information have been Saved Successfully.......!!!!!.');
;";
Page.ClientScript.RegisterStartupScript(this.GetType(),"myJScript1", script);
}

请帮助。

连接字符串：

1 2	<ADD name="sqlAddSave" connectionString="Data Source=(LocalDB)\v11.0;AttachDbFilename=\|DataDirectory\|\PPTableDisplay.mdf;Integrated Security=True" providerName="System.Data.SqlClient" />

例外

Exception Details: System.NullReferenceException was unhandled by user
code HResult=-2147467261 Message=Object reference not set to an
instance of an object. Source=TestCRole StackTrace: at
TestCRole._Default.Button8_Click(Object sender, EventArgs e) in
c:\Users\xxxxx\Documents\Visual Studio
2012\Projects\WindowsAzure2\TestCRole\Default.aspx.cs:line 60 at
System.Web.UI.WebControls.Button.RaisePostBackEvent(String
eventArgument) at System.Web.UI.Page.ProcessRequestMain(Boolean
includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
InnerException: