My b2evolution Version: 4.1.x

I want to remove the 'Welcome' cat but, although there's nothing displayed in the blog, it still says in Admin that there is a post in 'Welcome' through 'extra cat'.

Extra cat doesn't exist - previous posts in the forum are not relevant to this version of b2.

All posts have been deleted. Two other categories exist so it wouldn't be left with no cats.

Where can I find the mysterious post which doesn't show up anywhere! Also, what is extra cat and where is it please?