Unable to determine simulator device to boot. - Xcode

I just downloaded Xcode 6 & played with it a bit.

After switching back to previous version of Xcode and running in simulator, I keep getting this error.

Unable to determine simulator device to boot.

How do we resolve this?

Thanks in advance.

Make sure you have only one iOS Simulator instance running in the Dock.

There was a simple answer to this for me:

1. Open the simulator via Xcode -> Open Developer Tool -> iOS simulator (even if it can't launch, the app will be open).
2. With the simulator app open, go to Hardware -> Device -> Manage Devices.
3. Add any missing simulators with the plus button (all of mine were missing for some reason).

I've faced same issue and solved by below steps:

1) Move xcode6-beta into application folder

2) Restart mac and open xcode6-beta from application folder.

I had the same problem when running from Xcode 6 beta selecting iPhone 5s. If I choose iPhone 5 or "resizable iPhone" then my app launches fine.