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.