I know the unstable trait is supposed to swap his traits, but instead it just removes it. The notification went something like:

Your sim no longer has the following traits
-Dislikes children

Your sim now has the following traits
. (<-- it's just a weird period there and no trait)

As a result, my sim loses his dislikes children trait and now he has 1 empty slot.

This isn't a solution, more like a band-aid, but I think I would get Master Controller with the Cheats module and assign a new trait to the guy at this point.

If you have some dice or can go online to an RNG, you could make a sheet and randomly roll for a new trait. Or just pick one.

You probably will not even need a mod. Just enable testingcheats, shift-click on your Sim and chose the option in the menu to change traits.

@Ghost sdoj @Don Babilon

Thank you guys for the work arounds, I have nraas, so I'm using MC's "populate traits" for now. It's too bad because this trait would've been really interesting if it actually worked properly. Is there no way to fix it so it actually works?

I don't have that EP, so I don't have that trait, but I have seen indications that it usually works as advertised. Fixing it would involve going through (and actually doing each step, since it's a diagnostic tool) the whole Game Problem FAQ at http://simswiki.info/wiki.php?title..._3_Game_Problem to find out where in the process the trait is having trouble finding the one to change to, and then probably either doing the binary search dance through your CC to find out which one is the source of the problem or discovering you have a corrupt sim or a corrupt install. Any of that is a lot more work than just giving the guy a new trait once in a while. Of course, you could get lucky and have the problem fixed when you clear your caches.

@Ghost sdoj Thanks! The problem appears to have fixed itself though. It started working after removing the first time... weird, I didn't even clear caches or anything.

I'm glad it fixed itself. (Although now I'm starting to wonder if it wasn't trying to assign a Hidden trait, although I would expect that those are probably not supposed to be assigned. )