Sim dissappearing in secret lab (strangerville)

I was having the same problem with my sim disappearing in the secret lab and the furniture inside not appearing. I finally figured out that it was either a mod or a cheat that was conflicting. I had TwistedMexican's Secret Lab Build Buy mod installed and the always free build cheat turned on. I turned off the cheat and uninstalled the mod at the same time so I'm not sure which was causing the error but if you're sure a mod isn't conflicting, check to make sure free build is off.