Editor crash while saving asset during cook operation

It is possible to experience an editor crash in 5.5.4 when the cook process generates a warning that references an asset while the editor is saving. It seems that during a save operation, the code that updates the save progress bar will try to run background tasks. If those queued background tasks include a call to `FPackagingErrorHandler::AddMessageToMessageLog` to report an error or warning from the cook thread that references an asset path, then `AddMessageToMessageLog` will attempt to call `GetAsset()` on that `AssetData`, and during `StaticFindObjectFast` will generate a fatal error when `UE::IsSavingPackage(nullptr)` returns `true`.

1.) Add the following code to `UObject::PostLoad()` in order to generate a steady stream of warnings, making it more likely that the `AddMessageToMessageLog` task will be present during a save operation:

```
if (UPackage* Package = GetPackage())
{
UE_LOG(LogObj, Warning, TEXT("%s: This is a test warning."), *Package->GetName());
}
```

2.) Build the engine and open the Lyra project.

3.) From the Lyra editor, start a cook operation.

4.) Open an asset, and manually save it while the cook is ongoing and generating warnings. I was able to reproduce this issue consistently with the `/Effects/Textures/Flipbooks/SmokeSwirl_3_Flipbook_CHANNELPACK` asset.

Observe: The editor process will crash due to a fatal error generated in `StaticFindObjectFast`.
Expected: The editor process should not crash.

[Inline Frame] UnrealEditor-Core.dll!UE::Logging::Private::BasicFatalLog::__l2::<lambda>() Line 1103 C++
UnrealEditor-Core.dll!UE::Logging::Private::BasicFatalLog(const FLogCategoryBase & Category, const UE::Logging::Private::FStaticBasicLogRecord * Log, ...) Line 1103 C++
> UnrealEditor-CoreUObject.dll!StaticFindObjectFast(UClass * ObjectClass, UObject * ObjectPackage, FName ObjectName, bool bExactClass, EObjectFlags ExclusiveFlags, EInternalObjectFlags ExclusiveInternalFlags) Line 448 C++
UnrealEditor-UATHelper.dll!FAssetData::FastGetAsset(bool bLoad, TSet<FName,DefaultKeyFuncs<FName,0>,FDefaultSetAllocator> LoadTags) Line 522 C++
UnrealEditor-UATHelper.dll!FAssetData::GetAsset(TSet<FName,DefaultKeyFuncs<FName,0>,FDefaultSetAllocator> LoadTags) Line 566 C++
UnrealEditor-UATHelper.dll!FPackagingErrorHandler::AddMessageToMessageLog(FString MessageString, EMessageSeverity::Type MessageType) Line 234 C++
UnrealEditor-UATHelper.dll!UE::Core::Private::Tuple::TTupleBase<TIntegerSequence<unsigned int,0,1>,FString,enum EMessageSeverity::Type>::ApplyAfter<void (__cdecl*const &)(FString,enum EMessageSeverity::Type)>(void[Image Removed](FString, EMessageSeverity::Type) & Func) Line 317 C++
UnrealEditor-UATHelper.dll!TBaseStaticDelegateInstance<void __cdecl(void),FDefaultDelegateUserPolicy,FString,enum EMessageSeverity::Type>::ExecuteIfSafe() Line 779 C++
UnrealEditor-UATHelper.dll!TDelegate<void __cdecl(void),FDefaultDelegateUserPolicy>::ExecuteIfBound<void,0>() Line 634 C++
UnrealEditor-UATHelper.dll!TGraphTask<FSimpleDelegateGraphTask>::ExecuteTask() Line 634 C++
UnrealEditor-Core.dll!UE::Tasks::Private::FTaskBase::TryExecuteTask() Line 504 C++
[Inline Frame] UnrealEditor-Core.dll!FBaseGraphTask::Execute(TArray<FBaseGraphTask *,TSizedDefaultAllocator<32>> &) Line 482 C++
UnrealEditor-Core.dll!FNamedTaskThread::ProcessTasksNamedThread(int QueueIndex, bool bAllowStall) Line 779 C++
UnrealEditor-Core.dll!FNamedTaskThread::ProcessTasksUntilIdle(int QueueIndex) Line 679 C++
UnrealEditor-Engine.dll!FFrameEndSync::Sync(bool bFullSync) Line 13419 C++
UnrealEditor-UnrealEd.dll!TickSlate(TSharedPtr<SWindow,1> SlowTaskWindow) Line 437 C++
UnrealEditor-UnrealEd.dll!FFeedbackContextEditor::ProgressReported(const float TotalProgressInterp, FText DisplayMessage) Line 671 C++
UnrealEditor-Core.dll!FFeedbackContext::UpdateUI() Line 207 C++
UnrealEditor-Core.dll!FFeedbackContext::RequestUpdateUI(bool bForceUpdate) Line 197 C++
UnrealEditor-Core.dll!FSlowTask::EnterProgressFrame(float ExpectedWorkThisFrame, const FText & Text) Line 199 C++
UnrealEditor-Core.dll!FSlowTask::Destroy() Line 165 C++
[Inline Frame] UnrealEditor-CoreUObject.dll!FScopedSlowTask::

() Line 49 C++
UnrealEditor-CoreUObject.dll!`anonymous namespace'::WriteExports(FStructuredArchiveRecord & StructuredArchiveRoot, FSaveContext & SaveContext) Line 2265 C++
UnrealEditor-CoreUObject.dll!`anonymous namespace'::SaveHarvestedRealms(FSaveContext & SaveContext, ESaveRealm HarvestingContextToSave) Line 2916 C++
UnrealEditor-CoreUObject.dll!`anonymous namespace'::InnerSave(FSaveContext & SaveContext) Line 3093 C++
UnrealEditor-CoreUObject.dll!UPackage::Save2(UPackage * InPackage, UObject * InAsset, const wchar_t * InFilename, const FSavePackageArgs & SaveArgs) Line 3237 C++
UnrealEditor-CoreUObject.dll!UPackage::Save(UPackage * InOuter, UObject * InAsset, const wchar_t * Filename, const FSavePackageArgs & SaveArgs) Line 20 C++
UnrealEditor-UnrealEd.dll!UEditorEngine::Save(UPackage * InOuter, UObject * InAsset, const wchar_t * Filename, const FSavePackageArgs & InSaveArgs) Line 4428 C++
UnrealEditor-UnrealEd.dll!UEditorEngine::SavePackage(UPackage * InOuter, UObject * InAsset, const wchar_t * Filename, const FSavePackageArgs & SaveArgs) Line 4437 C++
UnrealEditor-UnrealEd.dll!UEditorEngine::Exec_Obj(const wchar_t * Str, FOutputDevice & Ar) Line 4659 C++
UnrealEditor-UnrealEd.dll!UEditorEngine::Exec_Editor(UWorld * InWorld, const wchar_t * Stream, FOutputDevice & Ar) Line 5732 C++
UnrealEditor-Core.dll!FExec::Exec(UWorld * InWorld, const wchar_t * Cmd, FOutputDevice & Ar) Line 18 C++
UnrealEditor-Engine.dll!UEngine::Exec(UWorld * InWorld, const wchar_t * Cmd, FOutputDevice & Ar) Line 5027 C++
UnrealEditor-UnrealEd.dll!UUnrealEdEngine::Exec(UWorld * InWorld, const wchar_t * Stream, FOutputDevice & Ar) Line 662 C++
[Inline Frame] UnrealEditor-UnrealEd.dll!SaveAsset(UPackage *) Line 1061 C++
UnrealEditor-UnrealEd.dll!InternalSavePackage(UPackage * PackageToSave, bool bUseDialog, bool & bOutPackageLocallyWritable, FOutputDevice & SaveOutput) Line 3569 C++
UnrealEditor-UnrealEd.dll!InternalPromptForCheckoutAndSave(const TArray<UPackage *,TSizedDefaultAllocator<32>> & FinalSaveList, bool bUseDialog, TArray<UPackage *,TSizedDefaultAllocator<32>> & OutFailedPackages) Line 4216 C++
UnrealEditor-UnrealEd.dll!FEditorFileUtils::PromptForCheckoutAndSave(const TArray<UPackage *,TSizedDefaultAllocator<32>> & InPackages, FEditorFileUtils::FPromptForCheckoutAndSaveParams & InOutParams) Line 4610 C++
UnrealEditor-UnrealEd.dll!FEditorFileUtils::PromptForCheckoutAndSave(const TArray<UPackage *,TSizedDefaultAllocator<32>> & InPackages, bool bCheckDirty, bool bPromptToSave, TArray<UPackage *,TSizedDefaultAllocator<32>> * OutFailedPackages, bool bAlreadyCheckedOut, bool bCanBeDeclined) Line 4689 C++
UnrealEditor-UnrealEd.dll!FAssetEditorToolkit::SaveAsset_Execute() Line 690 C++
[Inline Frame] UnrealEditor-UnrealEd.dll!Invoke(void(FAssetEditorToolkit::*)()) Line 66 C++
[Inline Frame] UnrealEditor-UnrealEd.dll!UE::Core::Private::Tuple::TTupleBase<TIntegerSequence<unsigned int>>::ApplyAfter(void(FAssetEditorToolkit::*)() &) Line 317 C++
UnrealEditor-UnrealEd.dll!TBaseSPMethodDelegateInstance<0,FAssetEditorToolkit,1,void __cdecl(void),FDefaultDelegateUserPolicy>::ExecuteIfSafe() Line 299 C++
[Inline Frame] UnrealEditor-Slate.dll!TDelegate<void __cdecl(void),FDefaultDelegateUserPolicy>::ExecuteIfBound() Line 634 C++
UnrealEditor-Slate.dll!FUIAction::Execute() Line 139 C++
UnrealEditor-Slate.dll!FUICommandList::ExecuteAction(const TSharedRef<FUICommandInfo const ,1> InUICommandInfo) Line 117 C++
UnrealEditor-Slate.dll!SToolBarButtonBlock::OnClicked() Line 496 C++
[Inline Frame] UnrealEditor-Slate.dll!Invoke(FReply(SToolBarButtonBlock::*)()) Line 66 C++
[Inline Frame] UnrealEditor-Slate.dll!UE::Core::Private::Tuple::TTupleBase<TIntegerSequence<unsigned int>>::ApplyAfter(FReply(SToolBarButtonBlock::*)() &) Line 317 C++
UnrealEditor-Slate.dll!TBaseSPMethodDelegateInstance<0,SToolBarButtonBlock,1,FReply __cdecl(void),FDefaultDelegateUserPolicy>::Execute() Line 282 C++
[Inline Frame] UnrealEditor-Slate.dll!TDelegate<FReply __cdecl(void),FDefaultDelegateUserPolicy>::Execute() Line 613 C++
UnrealEditor-Slate.dll!SButton::ExecuteOnClick() Line 467 C++
UnrealEditor-Slate.dll!SButton::OnMouseButtonUp(const FGeometry & MyGeometry, const FPointerEvent & MouseEvent) Line 392 C++
UnrealEditor-Slate.dll!FSlateApplication::RoutePointerUpEvent::__l8::<lambda_2>::operator()(const FArrangedWidget & TargetWidget, const FPointerEvent & Event) Line 5346 C++
UnrealEditor-Slate.dll!FEventRouter::Route<FReply,FEventRouter::FToLeafmostPolicy,FPointerEvent,`FSlateApplication::RoutePointerUpEvent'::`8'::<lambda_2>>(FSlateApplication * ThisApplication, FEventRouter::FToLeafmostPolicy RoutingPolicy, FPointerEvent EventCopy, const FSlateApplication::RoutePointerUpEvent::__l8::<lambda_2> & Lambda, ESlateDebuggingInputEvent DebuggingInputEvent) Line 456 C++
UnrealEditor-Slate.dll!FSlateApplication::RoutePointerUpEvent(const FWidgetPath & WidgetsUnderPointer, const FPointerEvent & PointerEvent) Line 5332 C++
UnrealEditor-Slate.dll!FSlateApplication::ProcessMouseButtonUpEvent(const FPointerEvent & MouseEvent) Line 5917 C++
UnrealEditor-Slate.dll!FSlateApplication::OnMouseUp(const EMouseButtons::Type Button, const UE::Math::TVector2<double> CursorPos) Line 5873 C++
UnrealEditor-ApplicationCore.dll!FWindowsApplication::ProcessDeferredMessage(const FDeferredWindowsMessage & DeferredMessage) Line 2271 C++
UnrealEditor-ApplicationCore.dll!FWindowsApplication::DeferMessage(TSharedPtr<FWindowsWindow,1> & NativeWindow, HWND__ * InHWnd, unsigned int InMessage, unsigned __int64 InWParam, __int64 InLParam, int MouseX, int MouseY, unsigned int RawInputFlags) Line 2783 C++
UnrealEditor-ApplicationCore.dll!FWindowsApplication::ProcessMessage(HWND__ * hwnd, unsigned int msg, unsigned __int64 wParam, __int64 lParam) Line 1944 C++
[Inline Frame] UnrealEditor-ApplicationCore.dll!WindowsApplication_WndProc(HWND__ *) Line 939 C++
UnrealEditor-ApplicationCore.dll!FWindowsApplication::AppWndProc(HWND__ * hwnd, unsigned int msg, unsigned __int64 wParam, __int64 lParam) Line 945 C++
user32.dll!UserCallWinProcCheckWow(struct ACTIVATION_CONTEXT *,int64 [Image Removed](struct tagWND *,unsigned int,unsigned __int64,int64),struct HWND_ *,enum WM_VALUE,unsigned __int64,_int64,void *,int) Unknown
user32.dll!DispatchMessageWorker() Unknown
[Inline Frame] UnrealEditor-ApplicationCore.dll!WinPumpMessages() Line 116 C++
UnrealEditor-ApplicationCore.dll!FWindowsPlatformApplicationMisc::PumpMessages(bool bFromMainLoop) Line 145 C++
UnrealEditor.exe!FEngineLoop::Tick() Line 5806 C++
[Inline Frame] UnrealEditor.exe!EngineTick() Line 69 C++
UnrealEditor.exe!GuardedMain(const wchar_t * CmdLine) Line 188 C++
UnrealEditor.exe!LaunchWindowsStartup(HINSTANCE__ * hInInstance, HINSTANCE__ * hPrevInstance, char * __formal, int nCmdShow, const wchar_t * CmdLine) Line 266 C++
UnrealEditor.exe!WinMain(HINSTANCE__ * hInInstance, HINSTANCE__ * hPrevInstance, char * pCmdLine, int nCmdShow) Line 317 C++
[Inline Frame] UnrealEditor.exe!invoke_main() Line 102 C++
UnrealEditor.exe!__scrt_common_main_seh() Line 288 C++
kernel32.dll!BaseThreadInitThunk() Unknown
ntdll.dll!RtlUserThreadStart() Unknown