PropertyGrid's CollectionEditor problem

Within a property grid application I am using a form that is derived from System.ComponentModel.Design.CollectionEditor to edit data in a collection. This form has an Add- and a Remove-Button. I am trying to display a custom form once "Add" is clicked (because I want to display a list of items from which the user may select one element in order to add it to the collection). For this reason I am overriding the method "SetItems" from CollectionEditor. It works fine so far, but the problem is that SetItems is called AGAIN once the user clicks the "OK" or "Cancel" button of the CollectionEditor-form. Are there any ideas how to solve this? Thanks a bunch...