Component/s: LLDB for Swift
I can reliably reproduce this with Xcode 10.3 and Swift 5.0.1 while debugging this open source project: https://github.com/ianyh/Amethyst
I'm on commit b478a849f7d583996ce8593b6a81c0c580017051. Setting a breakpoint on FocusTransitionCoordinator.focusScreen, then triggering it with option+shift+w and stepping around a bit will cause that crash. Inspecting variables in the debugger will also cause it.
Let me know if there's anything else I can provide to help fix this.