Listview.OnMouseMove löst AV aus, ich weiß nicht warum

[DeddyH]

Wie ist es so?

Delphi-Quellcode:

procedure TMain_Form.ListView1MouseMove(Sender: TObject;
  Shift: TShiftState; X, Y: Integer);
var
  Item: TListItem;
begin
  Item := ListView1.GetItemAt(0, Y);
  if Assigned(item) then begin
    Item := ListView1.GetItemAt(X, Y);
    if Assigned(Item) and (Item.Index <> ListView1.ItemIndex) then begin
      with Sender as TListView do begin
        if Assigned(Fotovorschau_am_Mauszeiger_Form) then begin
          Preview_Form.Close;
        end;
      end;
    end;
  end;
end;