I solved it following these steps:
- Open Xcode 6 beta
- Go to the menu Xcode > Open Developer Tool > iOS Simulator
- Even if an error dialog shows up, you still would have access to the iOS Simulator’s menu
- Select Hardware > Device > Manage Devices
- Click on the little + sign at the bottom
- Add (if missing) all the devices that you want, or delete and recreate the ones malfunctioning.
- If anyone of the simulator is not working then right click on it and delete it and then re-create it again