This is a trending crash coming out of the 4.18 Previews. Users have not provided any descriptions of their actions when the crash occurred, but at least two of the affected projects are being created for ARKit.
The callstack is similar to [Link Removed], but different enough that it seemed safest to log this separately.
Callstack from log
2017.10.02-15.28.43:345][401]LogSavePackage: Moving '../../../../../../vw/Documents/Unreal Projects/***/Saved/DemoMap_Auto322AF5188464984F26A66E3AC10B672A0.tmp' to '../../../../../../vw/Documents/Unreal Projects/***/Saved/Autosaves/Game/***/Maps/DemoMap_Auto3.umap' [2017.10.02-15.28.43:357][401]LogSavePackage: Display: Finished SavePackage ../../../../../../vw/Documents/Unreal Projects/nHover3/Saved/Autosaves/Game/***/Maps/DemoMap_Auto3.umap [2017.10.02-15.28.43:361][401]LogFileHelpers: Editor autosave (incl. sublevels) for '/Game/***/Maps/DemoMap' took 15.797 [2017.10.02-15.28.43:361][401]LogFileHelpers: Editor autosave (incl. sublevels) for all levels took 15.797 [2017.10.02-15.29.15:943][498]Lightmass on imac.lan: 5:37 min total, 1.25 sec importing, 3.13 sec setup, 22.9 sec photons, 5:09 min processing, 2 ms extra exporting [1026/1026 mappings]. Threads: 46:27 min total, 5:04 min processing. [2017.10.02-15.29.18:824][500]LogMac: Error: appError called: Assertion failed: TaskDataArray.Num() == Exporter->VolumetricLightmapTaskGuids.Num() [File:/Users/build/Build/++UE4+Release-4.18+Compile/Sync/Engine/Source/Editor/UnrealEd/Private/Lightmass/ImportVolumetricLightmap.cpp] [Line: 843] [2017.10.02-15.29.18:998][500]LogMac: === Critical error: === SIGSEGV: invalid attempt to access memory at address 0x3 [2017.10.02-15.29.18:998][500]LogMac: 0x0f7440db UE4Editor-Core.dylib!FGenericPlatformMisc::RaiseException(unsigned int) [UnknownFile]) 0x108f2ef9 UE4Editor-ApplicationCore.dylib!FMacErrorOutputDevice::Serialize(wchar_t const*, ELogVerbosity::Type, FName const&) [UnknownFile]) 0x0f9f1f6d UE4Editor-Core.dylib!FOutputDevice::Logf(wchar_t const*, ...) [UnknownFile]) 0x0f9685fb UE4Editor-Core.dylib!FDebug::AssertFailed(char const*, char const*, int, wchar_t const*, ...) [UnknownFile]) 0x174df9b8 UE4Editor-UnrealEd.dylib!FLightmassProcessor::ImportVolumetricLightmap() [UnknownFile]) 0x17504a98 UE4Editor-UnrealEd.dylib!FLightmassProcessor::CompleteRun() [UnknownFile]) 0x178cfdd9 UE4Editor-UnrealEd.dylib!FStaticLightingSystem::FinishLightmassProcess() [UnknownFile]) 0x178cf726 UE4Editor-UnrealEd.dylib!FStaticLightingManager::ProcessLightingData() [UnknownFile]) 0x178da19a UE4Editor-UnrealEd.dylib!FStaticLightingSystem::UpdateLightingBuild() [UnknownFile]) 0x178d9383 UE4Editor-UnrealEd.dylib!FStaticLightingManager::UpdateBuildLighting() [UnknownFile]) 0x178ee070 UE4Editor-UnrealEd.dylib!UEditorEngine::UpdateBuildLighting() [UnknownFile]) 0x0f63da75 UE4Editor!FEngineLoop::Tick() [UnknownFile]) 0x0f647252 UE4Editor!GuardedMain(wchar_t const*) [UnknownFile]) 0x0f654890 UE4Editor!-[UE4AppDelegate runGameThread:] [UnknownFile]) 0x0f8ee417 UE4Editor-Core.dylib!-[FCocoaGameThread main] [UnknownFile]) 0x440ad6d8 Foundation!__NSThread__start__() [UnknownFile]) 0x697e06c1 libsystem_pthread.dylib!_pthread_body() [UnknownFile]) 0x697e056d libsystem_pthread.dylib!_pthread_body() [UnknownFile]) 0x697dfc5d libsystem_pthread.dylib!thread_start() [UnknownFile])
1. Build UE4Editor
2. Run it with a project
3. Turn off your network interface (unplug the cable or use the UI)
4. Attempt to build the lighting
(Note: after one turn-off/turn-on cycle the editor may stop building the lighting even with the connection on. This is a limitation of this fix and it shouldn't be verify-failed unless it crashes).
SEGV_MAPERR at 0x3 UE4Editor-UnrealEd.dylib!FLightmassProcessor::ImportVolumetricLightmap() UE4Editor-UnrealEd.dylib!FLightmassProcessor::CompleteRun() UE4Editor-UnrealEd.dylib!FStaticLightingSystem::FinishLightmassProcess() UE4Editor-UnrealEd.dylib!FStaticLightingManager::ProcessLightingData() UE4Editor-UnrealEd.dylib!FStaticLightingSystem::UpdateLightingBuild() UE4Editor-UnrealEd.dylib!FStaticLightingManager::UpdateBuildLighting() UE4Editor-UnrealEd.dylib!UEditorEngine::UpdateBuildLighting() UE4Editor!FEngineLoop::Tick() UE4Editor!GuardedMain() UE4Editor!<Unknown> UE4Editor-Core.dylib!<Unknown> Foundation!Unknown() libsystem_pthread.dylib!<Unknown> libsystem_pthread.dylib!<Unknown> libsystem_pthread.dylib!<Unknown>
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-50658 in the post.