It crashes my hammer too.
Oh right. That.
There's something about the HL fgd that doesn't like to render cycler_sprites as models in Hammer, but ingame they'll be fine (I know. I use them :P)
Use furniture or cyclers or something as a placeholder, then switch to the CS fgd and change all of your props to cycler_sprites before the final compile.
(That or just change them and don't go into a 3D window).