Crash w. 'Serialized Class LinkerPlaceholderExportObject for a property of BlueprintGeneratedClass'

I was attempting to repro [Link Removed]. I created a new Actor BP, placed it in the level, added a ref to it to the level BP (instead of the projectile approach in the repro), and tried to save my map. I got a crash after the following warning:

LogProperty: Warning: Serialized Class /Script/CoreUObject.LinkerPlaceholderExportObject for a property of BlueprintGeneratedClass /Game/Developers/jamesgolding/UE-50215/Actor_01.Actor_01_C. Reference will be nullptred.
    Property = ObjectProperty /Game/Developers/jamesgolding/UE-50215/ReproMap.ReproMap_C:Actor_01_98_ExecuteUbergraph_ReproMap_RefProperty
    Item = LinkerPlaceholderExportObject /Game/Developers/jamesgolding/UE-50215/ReproMap.PLACEHOLDER-INST_of_Actor_01_C_0

I'm afraid I closed the debugger before I got the full call stack. When I re-opened the editor, it seems like my BP and level did save ok though... Resaving the map does not repro the issue.

Link to project:

[Link Removed]

1. Download and extract the linked project.
2. Open the project in editor.
3. Open the Actor_01 blueprint.
4. In the Spawn custom event logic, break the link between the Cube reference and Get Relative Transform.
5. Compile (it will create an error).
6. Create the link again.
7. Compile again.
8. Close the blueprint.
9. Select File > Open Level.
10. Open FirstPersonExampleMap.
11. When prompted, save both the blueprint and the level.

RESULT

Crash.

Assertion failed: (Index >= 0) & (Index < ArrayNum) [Link Removed] [Line: 610]
Array index out of bounds: 0 from an array of size 0

KERNELBASE.dll!UnknownFunction []
UE4Editor-ApplicationCore.dll!FWindowsErrorOutputDevice::Serialize() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\applicationcore\private\windows\windowserroroutputdevice.cpp:65]
UE4Editor-Core.dll!FOutputDevice::Logf__VA() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\core\private\misc\outputdevice.cpp:70]
UE4Editor-Core.dll!FDebug::AssertFailed() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\core\private\misc\assertionmacros.cpp:409]
UE4Editor-CoreUObject.dll!FLinkerPlaceholderObjectImpl::FindRawPlaceholderContainer() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\linkerplaceholderbase.cpp:214]
UE4Editor-CoreUObject.dll!FLinkerPlaceholderBase::AddReferencingPropertyValue() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\linkerplaceholderbase.cpp:417]
UE4Editor-CoreUObject.dll!UObjectProperty::SerializeItem() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\propertyobject.cpp:93]
UE4Editor-CoreUObject.dll!FPropertyTag::SerializeTaggedProperty() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\propertytag.cpp:188]
UE4Editor-CoreUObject.dll!UStruct::SerializeTaggedProperties() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\class.cpp:1075]
UE4Editor-CoreUObject.dll!UObject::SerializeScriptProperties() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\obj.cpp:1281]
UE4Editor-CoreUObject.dll!UObject::Serialize() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\obj.cpp:1215]
UE4Editor-Engine.dll!UEdGraphNode::Serialize() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\engine\private\edgraph\edgraphnode.cpp:414]
UE4Editor-BlueprintGraph.dll!UK2Node::Serialize() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\blueprintgraph\private\k2node.cpp:115]
UE4Editor-BlueprintGraph.dll!UK2Node_EditablePinBase::Serialize() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\blueprintgraph\private\k2node_editablepinbase.cpp:245]
UE4Editor-BlueprintGraph.dll!UK2Node_Event::Serialize() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\blueprintgraph\private\k2node_event.cpp:55]
UE4Editor-BlueprintGraph.dll!UK2Node_ActorBoundEvent::Serialize() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\blueprintgraph\private\k2node_actorboundevent.cpp:268]
UE4Editor-CoreUObject.dll!StaticDuplicateObjectEx() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\uobjectglobals.cpp:2056]
UE4Editor-UnrealEd.dll!FEdGraphUtilities::CloneGraph() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\unrealed\private\edgraphutilities.cpp:223]
UE4Editor-UnrealEd.dll!FEdGraphUtilities::CloneAndMergeGraphIn() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\unrealed\private\edgraphutilities.cpp:266]
UE4Editor-KismetCompiler.dll!FKismetCompilerContext::MergeUbergraphPagesIn() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\kismetcompiler\private\kismetcompiler.cpp:2789]
UE4Editor-KismetCompiler.dll!FKismetCompilerContext::CreateAndProcessUbergraph() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\kismetcompiler\private\kismetcompiler.cpp:3006]
UE4Editor-KismetCompiler.dll!FKismetCompilerContext::CreateFunctionList() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\kismetcompiler\private\kismetcompiler.cpp:3539]
UE4Editor-KismetCompiler.dll!FKismetCompilerContext::CompileClassLayout() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\kismetcompiler\private\kismetcompiler.cpp:3791]
UE4Editor-Kismet.dll!FBlueprintCompilationManagerImpl::FlushCompilationQueueImpl() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\kismet\private\blueprintcompilationmanager.cpp:712]
UE4Editor-Kismet.dll!FBlueprintCompilationManager::FlushCompilationQueue() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\kismet\private\blueprintcompilationmanager.cpp:1967]
UE4Editor-Engine.dll!UBlueprintGeneratedClass::ConditionalRecompileClass() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\engine\private\blueprintgeneratedclass.cpp:259]
UE4Editor-CoreUObject.dll!FScopedClassDependencyGather::~FScopedClassDependencyGather() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\blueprint\blueprintsupport.cpp:376]
UE4Editor-CoreUObject.dll!FLinkerLoad::CreateExport() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\linkerload.cpp:4024]
UE4Editor-CoreUObject.dll!FLinkerLoad::IndexToObject() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\linkerload.cpp:4241]
UE4Editor-CoreUObject.dll!FLinkerLoad::CreateExport() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\linkerload.cpp:3711]
UE4Editor-CoreUObject.dll!FLinkerLoad::CreateExportAndPreload() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\linkerload.cpp:2744]
UE4Editor-CoreUObject.dll!FLinkerLoad::LoadAllObjects() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\linkerload.cpp:2902]
UE4Editor-CoreUObject.dll!LoadPackageInternal() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\uobjectglobals.cpp:1322]
UE4Editor-CoreUObject.dll!LoadPackage() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\coreuobject\private\uobject\uobjectglobals.cpp:1430]
UE4Editor-UnrealEd.dll!UEditorEngine::Map_Load() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\unrealed\private\editorserver.cpp:2492]
UE4Editor-UnrealEd.dll!UEditorEngine::HandleMapCommand() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\unrealed\private\editorserver.cpp:5966]
UE4Editor-UnrealEd.dll!UEditorEngine::Exec() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\unrealed\private\editorserver.cpp:5443]
UE4Editor-UnrealEd.dll!UUnrealEdEngine::Exec() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\unrealed\private\unrealedsrv.cpp:672]
UE4Editor-UnrealEd.dll!FEditorFileUtils::LoadMap() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\unrealed\private\filehelpers.cpp:2229]
UE4Editor-UnrealEd.dll!`FEditorFileUtils::LoadMap'::`2'::FLocal::HandleLevelsChosen() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\unrealed\private\filehelpers.cpp:2063]
UE4Editor-UnrealEd.dll!TBaseStaticDelegateInstance<void __cdecl(TArray<FAssetData,FDefaultAllocator> const & __ptr64),bool * __ptr64>::ExecuteIfSafe() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:1027]
UE4Editor-UnrealEd.dll!`FEditorFileUtils::OpenLevelPickingDialog'::`2'::FLocal::OnLevelsSelected() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\unrealed\private\filehelpers.cpp:1844]
UE4Editor-UnrealEd.dll!TBaseStaticDelegateInstance<void __cdecl(TArray<FAssetData,FDefaultAllocator> const & __ptr64),TBaseDelegate<void,TArray<FAssetData,FDefaultAllocator> const & __ptr64> >::ExecuteIfSafe() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:1027]
UE4Editor-ContentBrowser.dll!SAssetDialog::OnConfirmClicked() [d:\build\++ue4+release-4.18+compile\sync\engine\source\editor\contentbrowser\private\sassetdialog.cpp:821]
UE4Editor-ContentBrowser.dll!TMemberFunctionCaller<SAssetDialog,FReply (__cdecl SAssetDialog::*)(void) __ptr64>::operator()<>() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\core\public\delegates\delegateinstanceinterface.h:165]
UE4Editor-ContentBrowser.dll!UE4Tuple_Private::TTupleImpl<TIntegerSequence<unsigned int> >::ApplyAfter<TMemberFunctionCaller<SAssetDialog,FReply (__cdecl SAssetDialog::*)(void) __ptr64> >() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\core\public\templates\tuple.h:497]
UE4Editor-ContentBrowser.dll!TBaseSPMethodDelegateInstance<0,SAssetDialog,0,FReply __cdecl(void)>::Execute() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:327]
UE4Editor-Slate.dll!TBaseDelegate<FReply>::Execute() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\core\public\delegates\delegatesignatureimpl.inl:537]
UE4Editor-Slate.dll!SButton::OnMouseButtonUp() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\slate\private\widgets\input\sbutton.cpp:282]
UE4Editor-Slate.dll!<lambda_1002768c627006711ef2f351a87ec0e7>::operator()() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5264]
UE4Editor-Slate.dll!FEventRouter::Route<FReply,FEventRouter::FToLeafmostPolicy,FPointerEvent,<lambda_1002768c627006711ef2f351a87ec0e7> >() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:234]
UE4Editor-Slate.dll!FSlateApplication::RoutePointerUpEvent() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5253]
UE4Editor-Slate.dll!FSlateApplication::ProcessMouseButtonUpEvent() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5755]
UE4Editor-Slate.dll!FSlateApplication::OnMouseUp() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5735]
UE4Editor-ApplicationCore.dll!FWindowsApplication::ProcessDeferredMessage() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:1725]
UE4Editor-ApplicationCore.dll!FWindowsApplication::DeferMessage() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:2171]
UE4Editor-ApplicationCore.dll!FWindowsApplication::ProcessMessage() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:888]
UE4Editor-ApplicationCore.dll!FWindowsApplication::AppWndProc() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:725]
USER32.dll!UnknownFunction []
USER32.dll!UnknownFunction []
UE4Editor-ApplicationCore.dll!FWindowsPlatformApplicationMisc::PumpMessages() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\applicationcore\private\windows\windowsplatformapplicationmisc.cpp:129]
UE4Editor.exe!FEngineLoop::Tick() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\launch\private\launchengineloop.cpp:3220]
UE4Editor.exe!GuardedMain() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\launch\private\launch.cpp:166]
UE4Editor.exe!GuardedMainWrapper() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:134]
UE4Editor.exe!WinMain() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:210]
UE4Editor.exe!__scrt_common_main_seh() [f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:253]
kernel32.dll!UnknownFunction []
ntdll.dll!UnknownFunction []
ntdll.dll!UnknownFunction []