[CrashReport] UE4Editor-CoreUObject.dylib!UPackage::Save() during COTF

"../../../../../../***/Documents/***/CrashResearch/***/Intermediate/Cache/5cd62d7f2147431bd176968cd9248b72RGBToPVROut.pvr"  -legacypvr -q pvrtcfast -f PVRTC1_2'
[2017.11.20-01.36.23:970][714]LogMac: Error: appError called: Assertion failed: !Export.ClassIndex.IsNull() [File:/Users/build/Build/++UE4+Release-4.18+Compile/Sync/Engine/Source/Runtime/CoreUObject/Private/UObject/SavePackage.cpp] [Line: 4630] 


[2017.11.20-01.36.24:690][714]LogTexture: Display: Building textures: T_FX_Squares (AutoPVRTC, 2048X2048)
[2017.11.20-01.36.24:690][714]LogTexture: Display: Building textures: T_FX_Wave (AutoPVRTC, 2048X512)
[2017.11.20-01.36.24:710][714]LogMac: === Critical error: ===
SIGSEGV: invalid attempt to access memory at address 0x3

[2017.11.20-01.36.24:710][714]LogMac: 0x0d81099b UE4Editor-Core.dylib!FGenericPlatformMisc::RaiseException(unsigned int)   [UnknownFile]) 
0x1364bc09 UE4Editor-ApplicationCore.dylib!FMacErrorOutputDevice::Serialize(wchar_t const*, ELogVerbosity::Type, FName const&)   [UnknownFile]) 
0x0dabec9d UE4Editor-Core.dylib!FOutputDevice::Logf(wchar_t const*, ...)   [UnknownFile]) 
0x0da34efb UE4Editor-Core.dylib!FDebug::AssertFailed(char const*, char const*, int, wchar_t const*, ...)   [UnknownFile]) 
0x0e74752a UE4Editor-CoreUObject.dylib!UPackage::Save(UPackage*, UObject*, EObjectFlags, wchar_t const*, FOutputDevice*, FLinkerLoad*, bool, bool, unsigned int, ITargetPlatform const*, FDateTime const&, bool)   [UnknownFile]) 
0x14e68b59 UE4Editor-UnrealEd.dylib!UEditorEngine::Save(UPackage*, UObject*, EObjectFlags, wchar_t const*, FOutputDevice*, FLinkerLoad*, bool, bool, unsigned int, ITargetPlatform const*, FDateTime const&, bool)   [UnknownFile]) 
0x14c1a8e5 UE4Editor-UnrealEd.dylib!UCookOnTheFlyServer::SaveCookedPackage(UPackage*, unsigned int, TArray<FName, FDefaultAllocator>&, TArray<FSavePackageResultStruct, FDefaultAllocator>&)   [UnknownFile]) 
0x14c0f1a2 UE4Editor-UnrealEd.dylib!UCookOnTheFlyServer::SaveCookedPackages(TArray<UPackage*, FDefaultAllocator>&, TArray<FName, FDefaultAllocator> const&, TArray<ITargetPlatform const*, FDefaultAllocator> const&, FCookerTimer&, int, unsigned int&, unsigned int&)   [UnknownFile]) 
0x14bfec5f UE4Editor-UnrealEd.dylib!UCookOnTheFlyServer::TickCookOnTheSide(float, unsigned int&, ECookTickFlags)   [UnknownFile]) 
0x14bfb640 UE4Editor-UnrealEd.dylib!UCookOnTheFlyServer::Tick(float)   [UnknownFile]) 
0x14e528b4 UE4Editor-UnrealEd.dylib!UEditorEngine::Tick(float, bool)   [UnknownFile]) 
0x15adcc2c UE4Editor-UnrealEd.dylib!UUnrealEdEngine::Tick(float, bool)   [UnknownFile]) 
0x0d704fc5 UE4Editor!FEngineLoop::Tick()   [UnknownFile]) 
0x0d70e7a2 UE4Editor!GuardedMain(wchar_t const*)   [UnknownFile]) 
0x0d71be40 UE4Editor!-[UE4AppDelegate runGameThread:]   [UnknownFile]) 
0x0d9bad17 UE4Editor-Core.dylib!-[FCocoaGameThread main]   [UnknownFile]) 
0x537fc6d8 Foundation!__NSThread__start__()   [UnknownFile]) 
0x78fb96c1 libsystem_pthread.dylib!_pthread_body()   [UnknownFile]) 
0x78fb956d libsystem_pthread.dylib!_pthread_body()   [UnknownFile]) 
0x78fb8c5d libsystem_pthread.dylib!thread_start()   [UnknownFile]) 

SEGV_MAPERR at 0x3

UE4Editor-CoreUObject.dylib!UPackage::Save()
UE4Editor-UnrealEd.dylib!UEditorEngine::Save()
UE4Editor-UnrealEd.dylib!UCookOnTheFlyServer::SaveCookedPackage()
UE4Editor-UnrealEd.dylib!UCookOnTheFlyServer::SaveCookedPackages()
UE4Editor-UnrealEd.dylib!UCookOnTheFlyServer::TickCookOnTheSide()
UE4Editor-UnrealEd.dylib!UCookOnTheFlyServer::Tick()
UE4Editor-UnrealEd.dylib!UEditorEngine::Tick()
UE4Editor-UnrealEd.dylib!UUnrealEdEngine::Tick()
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>