After installing Xcode 7 I can't build and run projects anymore as document fails to open

Question

EDIT: This question refers to teh pre-release version of Xcode 7, hence it may not be relevant if you are running the currently released version as some issues may have been

Answer 1

It seems this is a common issue. Found an answer over at Apple's developer forums:

https://forums.developer.apple.com/thread/11706

In case the link goes dead or you have no access, this is the accepted answer:

Correct Answer by seancook on Jul 21, 2015 5:58 PM

I experienced this, too, on El Capitan. NIBs wouldn't load, no Sims present, and unable to launch Simulators through the Xcode/Open Developer Tool/Simulator menu. After fighting it for a few hours I've finally got it working. Here's what worked for me (YMMV): I noticed a message in Console.app along the lines of the Simulator runtime not being code signed. I went to the path it described "/Library/Developer/CoreSimulator/Profiles/Runtimes" and removed the runtimes that I had there. They were old runtimes, so I assume that some check in Xcode now applies that didn't previously. Who knows?
After doing this, I could finally launch the Simulator via the Xcode/Open Developer Tool menu. Unfortunately, the Simulator still wasn't being shown as a run target in my workspace, so I kept spelunking... I then went into the Simulator's Hardware/Devices/Manage Devices menu and added a new iOS9 device. It didn't seem to do anything, so I quit out of Xcode and Simulator.app. I then re-started Simulator.app (via Xcode/ODT/Simulator) and lo and behold the expected iOS9 simulators were displayed. I closed and re-opened Xcode, changed the target OS for my project, and the Sims were finally displayed as run targets.

HTH.