Saving error

This message keeps popping up when I try to save my work on a map. I haven't saved the map on any other tabs or anything, so this obviously shouldn't be there, though when I opened the map it did say it needed to remove some deleted custom assets. I did delete a few old assets from my art page the other day, but not any that should've been on this map as far as I know. I'm not sure if the custom asset thing is related, but thought I should mention it.