The issue is split from https://phabricator.wikimedia.org/T89923. The issue is present in beta, test2, and production - see the attached screencast.

Re-arranging the categories(dragging them), then deleting them, and hitting 'Cancel' - causes errors when the Options-Categories dialog is re-opened.

1. Add three categories - AA, BB, CC
2. Move CC category - insert it in front of BB, then in front of AA (the sequence seems to be important)
3. Delete AA and BB - click 'Cancel'
4. Re-open Categories - CC category remains displayed.
5. Click CC to delete -

Error: Cannot remove detached item
and sometime

TypeError: this.categories[name] is undefined

The issue is not too serious- those 'detached' categories are not saved.