Empty item group linked to invoices


I had created a item-group structure then created some invoices with items that were on a specific item-group. I have since changed the item’s group to something else, but I can’t remove the old item-group which is now unused. It is in some way still linked to the invoices.

Either this is a bug or I am not seeing the logic in this. I have attached a few screenshots.

I hope someone can help me with this issue.