I have a custom module that I created.  In page load and not ispostback I get the values for the dropdown from a database.  The first time I submit it works fine.  If I then click the back button and submit again I get an error of "Object reference not set to an instance of an object."  It seems that for some reason it does not know what value is selected.  Can someone please help with this.  THank you.

 Private Sub butMC_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles butMC.Click
     Response.Redirect(NavigateURL() + "?agentid=" + Request.Params("AgentID") + "&mc=" + ddlMC.SelectedItem.Value)
End Sub

Yeah the items are there.  If I change the ddlMC.SelectedItem.Value to ddlMC.SelectedIndex then it comes back as -1 every time after the first time.

Try using this and seeing if you still get that error:

Response.Redirect(NavigateURL() + "?agentid=" + IIf(Request.Params("AgentID") Is Nothing, "", Request.Params("AgentID").ToString) + "&mc=" + ddlMC.SelectedItem.Value)

But this would be even better (performance-wise):

Response.Redirect(String.Concat(NavigateURL(), "?agentid=", IIf(Request.Params("AgentID") Is Nothing, "", Request.Params("AgentID").ToString).ToString, "&mc=", ddlMC.SelectedItem.Value))