Just updated from Xcode 6 Beta 3 to Beta 4 and everything seems to build fine but when the simulator tries to start up I get:
An error was encountered while running (Domain = com.apple.CoreSimulator.SimError, Code = 146)
It was building fine on Beta 3 so I'm assuming this might be a Beta 4 bug but thought I'd throw this out there just to make sure. Any ideas?