Xcode 9.3 Simulator Freezes

For a few days I had an issue where every so often the iOS simulator would freeze when booting up. It would just display an activity indicator. Then I saw this Tweet and now all is fixed!

Weird Xcode install issues with unbootable simulators?

xcrun simctl erase all

resolved it for me

— Jason Kneen (@jasonkneen) April 30, 2018