If you already upgraded XCode to newest 10.3 version, you may notice that opening XIBs created in older versions causes “Internal error” and UI elements are not visible. But there is simple solution for that.
Of course first thing to try is cleaning derived data, but if this won’t help, just close XCode, and execute in terminal:
sudo killall -9 com.apple.CoreSimulator.CoreSimulatorService
This will will all zombie simulator processes and everything will work again.