Okay, after doing a bit of trouble shooting, I think I've got a work around for that.

First off, if you use Ruler Designer you won't get that error. That's why I missed it in the first place. If you're playing normal then the problem will resolve itself once one of the Patrician families dies off. It appears to be purely a UI problem too, so everything works normally; it doesn't show the mansions on the Republic screen properly. You can still get to your mansion to upgrade it by manually scrolling through your holdings.

Sadly, there doesn't seem to be anything I can do on my end to fix it, but thanks for the heads up.

Shatari, I love this mod idea.

I tried using the mod and it is not working.
No mansions or houses appear in the republic tab.

I am using the latest update of ck2.
I am using a few small mods, nothing i think would interfere with yours though.

Alrighty, I've tested this out to make sure it works, and Steam seems to have uploaded it correctly. Let me know if there are any problems, but this is literally a single line of code so it shouldn't have any parts that can break.