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

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

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 ()