Here's some more interesting info we see in Crashlytics, IRT the same crash: Not possible to remove variable: 238: <unknown var (bug!) with engine as delegate:0x2812d0a80>{id: 37744} colIndex:59 from engine <NSISEngine: 0x11f5f8720>{ delegate:0x11f5f4f80 EngineVars: 0: objective{id: 35029} rowIndex:0 1: app.UserAvatarView:0x11f5f6440.Width{id: 35020} rowIndex:9 2: app.UserAvatarView:0x11f5f6440.Height{id: 35017} colIndex:1073741871 3: 0x282ecf9c0.marker{id: 35019} colIndex:1 4: app.UserAvatarView:0x11f5f6d00.Width{id: 35024} rowIndex:1073741825 5: app.UserAvatarView:0x11f5f6d00.Height{id: 35022} rowIndex:2 7: app.UserAvatarView:0x11f5f75c0.Width{id: 35028} colIndex:2 8: app.UserAvatarView:0x11f5f75c0.Height{id: 35026} rowIndex:3 10: UIStackView:0x11f5f62b0.Height{id: 35018} rowIndex:1073741824 11: 0x282ecf840.marker{id: 35016} colIndex:1073741825 12: 0x282e2b660.marker{id: 35021} colIndex:1073741826 13: 0x282e2bc00.marker{id: 35025} colIndex:1073741827 14: UIStackView:0x11f5f6120.minX{id: 35032} rowIndex:4 15: app.UsersSnippetView:0x11f5f5f60.minX{id: 35033} rowIndex:5 16: 0x282e5bea0.marker{id: 35031} colIndex:6 17: UIStackView:0x11f5f6120.minY{id: 35035} rowIndex:1073741827 18: 0x2821c4540.marker{id: 37785} colIndex:70 19: 0x282e59e00.marker{id: 35034} colIndex:1073741829 20: UIStackView:0x11f5f6120.Width{id: 35038} colIndex:4 21: app.UsersSnippetView:0x11f5f5f60.Width{id: 35039} rowIndex:53 22: 0x282e59d40.marker{id: 35037} colIndex:8 23: UIStackView:0x11f5f6120.Height{id: 35041} rowIndex:1073741885 24: app.UsersSnippetView:0x11f5f5f60.Height{id: 35042} rowIndex:1073741828 25: 0x282e59da0.marker{id: 35040} colIndex:1073741831 26: UIImageView:0x11f5f6960.Width{id: 37497} rowIndex:6 27: 0x2821fb540.posErrorMarker{id: 37493} colIndex:9 28: 0x2821fb540.negError{id: 37494} rowIndex:13 29: UIImageView:0x11f5f6960.Height{id: 37498} rowIndex:1073741829 30: 0x2821fb6c0.posErrorMarker{id: 37495} colIndex:1073741832 31: 0x2821fb6c0.negError{id: 37496} rowIndex:1073741836 32: UIView:0x11f5f66

Thanks for the 'Include Configuration Intent' hint, that does help for running the extension. But, I can't get Xcode Previews to work at all (not even with the WWDC-provided sample code). Getting errors similar to: RemoteHumanReadableError PreviewAgentError: Statically rendering previews is not supported on iOS simulators Any ideas?