I can delete the box (I have), but it appeared immediately after I deleted a woman’s husband. It was ‘married’ to that woman even though I hadn’t married her to anyone else yet. I don’t think I had just added anyone to the tree without adding details, because in this situation the box usually will just say ‘Partner of X’.
There don’t seem to be any problems because of this: I can still use the tree as normal. I am slightly concerned if this could be some sort of bug though.
Thanks for your reply!