Hmm. 
It is unusual to be able to find it one way and not the other.
The rails console can be a bit of a sharp knife so it’d be best to figure out what exactly has gone wrong just to minimise any accidents/knock on effects.
How was this category deleted in the first place?
(Just in case the issue isn’t caused by this stray topic, I’ll throw in the basics of checking safe mode and seeing if there’s any more info about the error in your YourSite/logs page too)
Though I don’t know if your data is in a state where the category delete catch-all could help? Bulk delete all topics in a category