I believe it is because you are using an implicitly unwrapped optional value with Int!, which means that you are guaranteeing it will have a value before value is accessed. To prevent the crash you should just assign an initial value to it. var value: Int! = 0 That may or may not work for your situation.

I submitted another feedback: FB9110670 "Accessibility Inspector Repeatedly Disconnects from iPhone Preventing Use" I cannot use it on actual apps without frequent disconnects. I can only hover over apps in Simulator using my Mac's All Processes, if I select the simulator it doesn't show highlight's around elements. I can only hover, and then have to watch the inspector properties as they change without size context of the element.

It looks like if I let the MacBook Pro monitor go back to sleep that the issue is resolved when I log in again (Using Touch ID worked this last time). Sometimes unplugging my monitor has fixed the issue, but that doesn't seem to work most of the time. (Or something else resolved it)