ListView and Edit/Update command

Hi,

I have a ListView and I want to implement the insert and update functionality.
Insert works great! But the update procedure doesn't work because in the update procedure I cannot access my controls:

        protected void btn_Update_Click(object sender, EventArgs evt)
        {
            String typ = ((DropDownList)lvw_Zuordnung.EditItem.FindControl("lst_Typ")).SelectedValue;

            ---> NullReferenceException
        }

The same works great in the insert scenario (with ListView.InsertItem),

What can I do?

Thank you very much
Magnus