Xcode error "Could not find Developer Disk Image"

For people who would have similar problems in the future, beware that this problem is fundamentally rooted in the mismatch of your iOS version and Xcode version.

Check the compatibility of iOS and Xcode.

I personally downloaded Xcode 6.4 beta and 7.0 beta and I was very happy to find the solution by searching "8.4" inside the application folder of the 6.4 beta. By doing this, I found the folder 8.4 (12H4125a) containing the iOS 8.4 image and I copied this folder to the same path of the 7.0 beta. The path is the following:

/Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport 

When you will reopen Xcode 7 and choose your device, there will be an error message; just click on fix issue and that should do it!