Garry's Mod 13.12.06 Bug.

For those of you that are experiencing crashes frequently, don’t panic. It’s none of your addon’s fault. Or anything you did to Garry’s Mod. Garry’s Mod update 13.12.06 has a bug which crashes you everytime you press z (Undo Button). It may work at times, but every DUPES are unable to be undoned. Hope this helps. :3

PS: Try avoiding your hand from pressing Z, plus theres addon which can clean your builds. Use remover instead of undo button C;

Works for me?

It is fixed in Dev version.