If you ever get an error with your XCode like this:
You can download the various device support from this repo I found: https://github.com/filsv/iPhoneOSDeviceSupport
Then place them in your device support folder:
Just rename them to their respective version number. e.g. 11.3 (15E5178d) to 11.3
You should be good to go after restarting Xcode, all the best.