Unable to run app in Simulator: Xcode beta 6 iOS 8

• Make sure you choose Xcode 6 as Command Line Tools in Preferences > Locations Section

• Make sure you open Xcode 6 from Applications not from the setup file.

• Finally restarting Mac solve the problem in my case.

I solved it following these steps:

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

After I found some solutions on the web, a combination of procedures worked for me:

1. Close Xcode and iOS Simulator (and all related applications)

2. Install (or reinstall) Xcode6-Beta to Applications folder (direct from the .dmg file)

3. Open Xcode6-Beta and go to Xcode -> Preferences -> Locations -> Command Line Tools and select "Xcode 6.0" (be sure that you've selected the one in Applications folder)

4. Launch the iOS Simulator and go to iOS Simulator -> Reset Content and Settings...

5. Close all the applications and restart your mac (I unchecked "Reopen windows...")

6. Open Xcode6-beta and test the iOS Simulator

Good luck