The editor will now fail this assert and exit:
check(IsInGameThread() || IsInGarbageCollectorThread());
Expected result: the morph target should be removed from the mesh, and should still be gone after restarting the editor and loading the mesh again.
UnrealEditor-CoreUObject.dll!FUObjectArray::FreeUObjectIndex(UObjectBase * Object) Line 291 UnrealEditor-CoreUObject.dll!FUObjectArray::FreeUObjectIndex(UObjectBase * Object) Line 291 UnrealEditor-CoreUObject.dll!UObjectBase::~UObjectBase() Line 148 [Inline Frame] UnrealEditor-Engine.dll!UMorphTarget::\{dtor}() Line 94 UnrealEditor-Engine.dll!UMorphTarget::`vector deleting destructor'(unsigned int) UnrealEditor-CoreUObject.dll!StaticAllocateObject(const UClass * InClass, UObject * InOuter, FName InName, EObjectFlags InFlags, EInternalObjectFlags InternalSetFlags, bool bCanRecycleSubobjects, bool * bOutRecycledSubobject, UPackage * ExternalPackage) Line 3300 UnrealEditor-CoreUObject.dll!StaticConstructObject_Internal(const FStaticConstructObjectParameters & Params) Line 4217 [Inline Frame] UnrealEditor-Engine.dll!NewObject(UObject *) Line 1656 UnrealEditor-Engine.dll!FFinishBuildMorphTargetData::ApplyEditorData(USkeletalMesh * SkeletalMesh, bool bIsSerializeSaving) Line 273 UnrealEditor-Engine.dll!FSkeletalMeshRenderData::Cache(const ITargetPlatform * TargetPlatform, USkinnedAsset * Owner, FSkinnedAssetCompilationContext * ContextPtr) Line 325 UnrealEditor-Engine.dll!USkeletalMesh::CacheDerivedData(FSkinnedAssetCompilationContext * ContextPtr) Line 4301 UnrealEditor-Engine.dll!USkeletalMesh::ExecuteBuildInternal(FSkinnedAssetBuildContext & Context) Line 1939 UnrealEditor-Engine.dll!FSkinnedAssetAsyncBuildWorker::DoWork() Line 47 UnrealEditor-Engine.dll!FAsyncTaskBase::DoWork() Line 285 UnrealEditor-Engine.dll!FAsyncTaskBase::DoThreadedWork() Line 309 UnrealEditor-Core.dll!FQueuedThreadPoolWrapper::FScheduledWork::DoThreadedWork() Line 128 UnrealEditor-Core.dll!FQueuedThreadPoolWrapper::FScheduledWork::DoThreadedWork() Line 128 [Inline Frame] UnrealEditor-Core.dll!FQueuedLowLevelThreadPool::AddQueuedWork::__l2::<lambda_9ed0233657edd7a72d2d0baa6df6647d>::operator()() Line 467 UnrealEditor-Core.dll!LowLevelTasks::FTask::Init::__l15::<lambda>(const bool bNotCanceled) Line 504 [Inline Frame] UnrealEditor-Core.dll!Invoke(LowLevelTasks::FTask::Init::__l15::LowLevelTasks::FTask * <lambda>(const bool) &) Line 47 [Inline Frame] UnrealEditor-Core.dll!LowLevelTasks::TTaskDelegate<LowLevelTasks::FTask * __cdecl(bool),48>::TTaskDelegateImpl<LowLevelTasks::FTask * <lambda>(const bool),0>::Call(void *) Line 162 UnrealEditor-Core.dll!LowLevelTasks::TTaskDelegate<LowLevelTasks::FTask * __cdecl(bool),48>::TTaskDelegateImpl<LowLevelTasks::FTask * <lambda>(const bool),0>::CallAndMove(LowLevelTasks::TTaskDelegate<LowLevelTasks::FTask * __cdecl(bool),48> & Destination, void * InlineData, unsigned int DestInlineSize, bool <Params_0>) Line 171 [Inline Frame] UnrealEditor-Core.dll!LowLevelTasks::TTaskDelegate<LowLevelTasks::FTask * __cdecl(bool),48>::CallAndMove(LowLevelTasks::TTaskDelegate<LowLevelTasks::FTask * __cdecl(bool),48> &) Line 308 UnrealEditor-Core.dll!LowLevelTasks::FTask::ExecuteTask() Line 656 UnrealEditor-Core.dll!LowLevelTasks::FScheduler::ExecuteTask(LowLevelTasks::FTask * & InOutTask) Line 162 UnrealEditor-Core.dll!LowLevelTasks::FScheduler::TryExecuteTaskFrom<&LowLevelTasks::TLocalQueueRegistry<1024>::TLocalQueue::DequeueGlobal,0>(LowLevelTasks::TLocalQueueRegistry<1024>::TLocalQueue * Queue, LowLevelTasks::TLocalQueueRegistry<1024>::FOutOfWork & OutOfWork, bool bPermitBackgroundWork, bool bDisableThrottleStealing) Line 361 UnrealEditor-Core.dll!LowLevelTasks::FScheduler::WorkerMain(LowLevelTasks::FSleepEvent * WorkerEvent, LowLevelTasks::TLocalQueueRegistry<1024>::TLocalQueue * ExternalWorkerLocalQueue, unsigned int WaitCycles, bool bPermitBackgroundWork) Line 402 [Inline Frame] UnrealEditor-Core.dll!LowLevelTasks::FScheduler::CreateWorker::__l2::<lambda_0a548c7e497de3cc77a9e48080e1524f>::operator()() Line 90 [Inline Frame] UnrealEditor-Core.dll!Invoke(LowLevelTasks::FScheduler::CreateWorker::__l2::<lambda_0a548c7e497de3cc77a9e48080e1524f> &) Line 47 UnrealEditor-Core.dll!UE::Core::Private::Function::TFunctionRefCaller<<lambda_0a548c7e497de3cc77a9e48080e1524f>,void __cdecl(void)>::Call(void * Obj) Line 475 [Inline Frame] UnrealEditor-Core.dll!UE::Core::Private::Function::TFunctionRefBase<UE::Core::Private::Function::TFunctionStorage<1>,void __cdecl(void)>::operator()() Line 602 UnrealEditor-Core.dll!FThreadImpl::Run() Line 67 UnrealEditor-Core.dll!FRunnableThreadWin::Run() Line 149 UnrealEditor-Core.dll!FRunnableThreadWin::GuardedRun() Line 71
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-168068 in the post.