Where is this occuring in my code ? Object reference not set to an instance of an object.

Hi;


 I get the following error when I click on a button.
The button pops up a panel which contains a details view control.
Object reference not set to an instance of an object.
 
And here is the button click code:

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click Button1.Enabled = False DropDownList12.Visible = False DropDownList13.Visible = False Dim compareUID As String = Web.Configuration.WebConfigurationManager.AppSettings("mgr") ...