Happening for me as well and restarting XCode not always solves the issue, very random. If I enable breakpoints on exceptions this is the stack trace when it happens: #0 0x000000019e7be234 in objc_exception_throw () #1 0x000000019e9acc4c in -[NSObject(NSObject) doesNotRecognizeSelector:] () #2 0x000000019ea9db24 in _forwarding_ () #3 0x000000019ea9fa9c in _CF_forwarding_prep_0 () #4 0x00000001de71a1c4 in -[EXExtensionContextImplementation _willPerformHostCallback:] () #5 0x00000001de71c4c4 in __112-[EXConcreteExtensionContextVendor _beginRequestWithExtensionItems:listenerEndpoint:withContextUUID:completion:]_block_invoke () #6 0x000000019e75a6f0 in _dispatch_call_block_and_release () #7 0x000000019e75b6d8 in _dispatch_client_callout () #8 0x000000019e704cf0 in _dispatch_lane_serial_drain$VARIANT$mp () #9 0x000000019e7057cc in _dispatch_lane_invoke$VARIANT$mp () #10 0x000000019e70f170 in _dispatch_workloop_worker_thread () #11 0x000000019e7b25a4 in _pthread_wqthread ()

I created feedback FB7851894 for this. Hope it gets resolved soon, it's really painful to debug my extension now.

I still don't see a link to request a lab appointment. This page just points to generic forum pages: https://developer.apple.com/news/?id=9njsyaod