Edit: I think I finally tracked this down actually! I wrote a short devlog about the bug: https://rmkubik.itch.io/island-maker/devlog/386701/fixed-undefined-undefined-bug
Thank you for helping debug and for checking the game out. :-)
I’m still trying to get to the bottom of this bug! Do you happen to remember what you placed right before you had the undefined show up? The boat? A windmill?