Simultaneous accesses to 0x1c0a7f0f8, but modification requires exclusive access error on Xcode 9 beta 4
I think this 'bug' may be a Swift 4 'feature', specifically something they call 'Exclusive access to Memory'.
Check out this WWDC video. Around the 50 minute mark, the long-haired speaker explains it.
https://developer.apple.com/videos/play/wwdc2017/402/?time=233
You could try turning the thread sanitizer off in your scheme settings if you're happy to ignore it. However, the debugger is trying to tell you about a subtle threading issue so it's probably a better use of your time to try to figure out why you've got something writing to your array at the same time it's being read from.
Under the target's Build Settings. Select No Enforcement for Exclusive Access to Memory from Swift Compiler - Code Generation