Object reference not set to an instance of an object.

I still can't crack this. It's ok on the first page_load because I cancelled the sqldatasource selecting event, but if the user then selects the empty 'choose a page' value in the dropdownlist then I get error. How do I put exception to catch this?
Thanks
Andy



Imports System.Data
Imports System.Data.SqlClient
Partial Class Admin_Default
    Inherits System.Web.UI.Page
    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    End Sub
    Protected Sub DropDownList1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DropDownList1.SelectedIndexChanged
        If DropDownList1.SelectedValue IsNot Nothing Then
            Dim dv As DataView
    & ...