I ran into a problem this morning when I tried to launch a React Native app I've been working on in the iOS Simulator using the command:
$ react-native run-ios
The build would fail completely and return the error message Could not parse the simulator list output. That's not good! Turns out an updated version of Xcode had been downloaded since I last ran the command, but I hadn't launched Xcode itself. When I launched Xcode it prompted me to download additional tools, and when this process was completed I was back in business.