To reproduce: run the attached sample project. Select an item and press F2, select a new item from the drop-down. You will encounter an error because the ActiveEditor is now null in the ItemValueChanging event.

Workaround: in the EditorInitialized event you can subscribe to the RadDropDownListElement.SelectedIndexChanging event:

private void radListView1_EditorInitialized(object sender, ListViewItemEditorInitializedEventArgs e)
{
    ListViewDropDownListEditor editor = e.Editor as ListViewDropDownListEditor;
    if (editor==null)
    {
        return;
    }
    RadDropDownListElement dropdown = (RadDropDownListElement)editor.EditorElement;
    dropdown.SelectedIndexChanging-=dropdown_SelectedIndexChanging;
    dropdown.SelectedIndexChanging+=dropdown_SelectedIndexChanging;
}