Thanks, I think the problem is the filter=append. This is probably intentional, but it looks like the piece that would lead to the notice (append isn't a real filter name).
If you can figure out the steps to reproduce, that would still be helpful (so I can confirm any fix). Judging by your referrer description, I'm guessing:
1. On category page view, click Add Listing > Add existing page.
2. Filter pages by category.
3. E_NOTICE