There is a setting on some objectives that lets you set them to spawn after another one.
You have to have another objective in the mission that is mandatory for mission completion, and then set the second objective to spawn based on the first.
In your case that would be setting the Collection to spawn after the boss is defeated. (Not entirely sure you can do this with a Collection objective, haven't tried it myself.)
Note: This method of chaining objectives CAN get irritating when playing a mission. Use it carefully.
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Nope... I found what you were talking about, but Collection objectives don't have the 'this mission goal is created when...' setting option. 
Dang...this was going to be the only chained objective, but logic demands it be chained. Now I'll have to figure out some way to ignore the logic or something. Maybe if I set Boss to spawn Middle and Safe to spawn in the Back, and have the game actually spawn them that way (it keeps putting things out of order for me for some reason I've yet to fathom), it will work. Won't be the way I wanted, but it'll be logical, at least.
I have a collection item I want to only be interactable (or visible, perhaps?) AFTER a previous mission objective is reached (killing boss). Basically, the mission objectives are: you have to defeat the boss to get the combination for the safe and then get the item out of the safe.
It wasn't until I ran my mission and found the safe and got the item out of it *before* I found the boss and defeated him for the code (to the safe I'd already opened)...that I knew there was a problem here.
Is there any possible work-around? Since we can't set specific locations on our maps, and we can't do like some dev-made maps do and put mission objectives behind locked doors, I don't know of any way I can achieve this. Anyone have an idea that might work?