Hello I have the same issue here.I am using a 2012 macbook pro - witth all the current versions of osx and xcode etc.I have two account set up on my MBP. I have been using Xcode on my main account and experienced the Preview failure in Xcode.I also have a Macmini 2012 all up to date and there is no problem with Xcode Preview sor far.I opened my second user account on my MBp 2012 and ran Xcode from that account and all is fine - preview works ok.So it looks the hardware is ok and the OS and Xcode is ok. But for some reason my Main user account can fnd the necssary links to show the preview.I would suggest, as a work round, creating an addition user account and run Xcode from there and cheeck if that does the trick for you.(I was thinking of uninstalling xcode and then removing all the associated xcode files and then reinstalling xcode - but this work round will be ok for me).RegardsK