I was testing the new spriteKit Particle emitters, and have somehow made configurations in a project such that xCode crashes when it opens the project. I've restarted and tried it on my backup machine. This is happening in both the Xcode 6 beta, and the production 6.0.1 release.
I pushed the project source to github if anyone wants to take a look at it.
UPDATE from Apple:
Apple Developer Relations
08-Oct-2014 07:55 AM
Engineering has provided the following information:
We believe this issue has been addressed in the latest Xcode 6.1 beta.
This pre-release version of the Xcode 6.1 developer tools is required when developing for OS X Yosemite, and includes the Swift programming language. Xcode 6.1 beta runs on OS X Mavericks and OS X Yosemite Developer Preview.
Please test with this release, and update your bug report with the results.
Xcode 6.1 beta - Build 6A1046b
https://developer.apple.com/xcode/downloads/
Please let us know if that resolves the issue for you by updating your bug report.