[CrashReport] Assertion failed: !Export.ClassIndex.IsNull() | UPackage::Save()

 4579       				for (int32 i = 0; i < Linker->ExportMap.Num(); i++)
 4580       				{
 4581       					FObjectExport& Export = Linker->ExportMap[i];
 4582       					if (Export.Object)
 4583       					{
 4584       						// Set class index.
 4585       						// If this is *exactly* a UClass, store null instead; for anything else, including UClass-derived classes, map it
 4586       						UClass* ObjClass = Export.Object->GetClass();
 4587       						if (ObjClass != UClass::StaticClass())
 4588       						{
 4589       							Export.ClassIndex = Linker->MapObject(ObjClass);
 4590       							check(!Export.ClassIndex.IsNull());
 4591       						}
 4592 ***** 						else
 4593       						{
 4594       							Export.ClassIndex = FPackageIndex();
 4595       						}
 4596       
 4597       						if (IsEventDrivenLoaderEnabledInCookedBuilds() && TargetPlatform)
 4598       						{
 4599       							UObject* Archetype = Export.Object->GetArchetype();
 4600       							check(Archetype);
 4601       							check(Archetype->IsA(Export.Object->HasAnyFlags(RF_ClassDefaultObject) ? ObjClass->GetSuperClass() : ObjClass));
 4602       							Export.TemplateIndex = Linker->MapObject(Archetype);
 4603       							UE_CLOG(Export.TemplateIndex.IsNull(), LogSavePackage, Fatal, TEXT("%s was an archetype of %s but returned a null index mapping the object."), *Archetype->GetFullName(), *Export.Object->GetFullName());
 4604       							check(!Export.TemplateIndex.IsNull());
 4605       						}