Can you attach a screenshot of the updater window and "(null)" indication. It will help understanding. Make sure you complete your files backup before anything else, just in case, but probably you have Z16 remaining in its pre-update state.
I have not used updater myself to go from Z16 to Z17 myself yet.
@337harvey probably has most experience of the updater from Beta onwards, so may contribute some advice re "(null)" and seemingly stuck updater.
EDIT: See experience of another updater user here: Zorin 16 Pro to zorin Pro 17 update
Maybe use System Monitor to see if updater is indeed active or not.