Hiding "end activity" if no inventory items added and populating field according to added items

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Hey, 

First question:
We have a situation that we don't want technicians to complete activities without adding any inventory items. So basically if ActualAmount = 0, hide "end activity" button. 
Is there a way to do this without adding extra properties?

Currently we've done this so that when ending activity, it just shows a checkbox which asks technician "have they added all the needed items", but as you know this isn't enough because it doesn't actually check that.

I noticed that there was similar issue about showing inventory.properties in activity end screen and it was already posted to Idea Lab, which was great!