Editor crashes while saving or closing after heightmap import

Editor crashes while attempting to save or close project after importing a landscape from a heightmap.

Frequency: 2/2

Crashreporter: [Link Removed]

Regression (yes) issue (does not) occur in 4.11.2

1. Download attached .png image
2. Open Editor (any project)
3. Modes>Landscape>Import from File>downloaded .PNG>Create
4. Save map (may crash here)
5. If editor remains open, attempt to close editor using the "X" button at the top right of the screen.

Results

Editor crashes

Expected

Editor closes

MachineId:409D523543AE785961A2788C698A2F90
EpicAccountId:89e014249c0a480eb18ea5b683d2eb3c

Assertion failed: Result == FPropertyAccess::Success [Link Removed] [Line: 989]

UE4Editor_UnrealEd!TObjectIterator<UObject>::operator++() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\coreuobject\public\uobject\uobjectiterator.h:289]
UE4Editor_UnrealEd!PackageTools::GetFilteredPackageList() [d:\build\++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\packagetools.cpp:64]
UE4Editor_UnrealEd!PackageTools::CheckForReferencesToExternalPackages() [d:\build\++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\packagetools.cpp:536]
UE4Editor_UnrealEd!UEditorEngine::PackageUsingExternalObjects() [d:\build\++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\playlevel.cpp:3567]
UE4Editor_UnrealEd!FEditorFileUtils::SaveMap() [d:\build\++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\filehelpers.cpp:2213]
UE4Editor_UnrealEd!SaveAsImplementation() [d:\build\++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\filehelpers.cpp:863]
UE4Editor_UnrealEd!FEditorFileUtils::SaveLevel() [d:\build\++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\filehelpers.cpp:1090]
UE4Editor_UnrealEd!FEditorFileUtils::SaveCurrentLevel() [d:\build\++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\filehelpers.cpp:2915]
UE4Editor_LevelEditor!TBaseStaticDelegateInstance<void __cdecl(void)>::ExecuteIfSafe() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:1021]
UE4Editor_Slate!FUICommandList::ExecuteAction() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\private\framework\commands\uicommandlist.cpp:87]
UE4Editor_Slate!SToolBarButtonBlock::OnClicked() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\private\framework\multibox\stoolbarbuttonblock.cpp:300]
UE4Editor_Slate!TMemberFunctionCaller<SToolBarButtonBlock,FReply (__cdecl SToolBarButtonBlock::*)(void) __ptr64>::operator()<>() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\public\delegates\delegateinstanceinterface_variadics.h:161]
UE4Editor_Slate!TTupleImpl<TIntegerSequence<unsigned int> >::ApplyAfter_ExplicitReturnType<FReply,TMemberFunctionCaller<SToolBarButtonBlock,FReply (__cdecl SToolBarButtonBlock::*)(void) __ptr64> >() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\public\delegates\tuple.h:128]
UE4Editor_Slate!TBaseSPMethodDelegateInstance<0,SToolBarButtonBlock,0,FReply __cdecl(void)>::Execute() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:321]
UE4Editor_Slate!TBaseDelegate<FReply>::Execute() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\public\delegates\delegatesignatureimpl_variadics.inl:521]
UE4Editor_Slate!SButton::OnMouseButtonUp() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\private\widgets\input\sbutton.cpp:275]
UE4Editor_Slate!<lambda_8d0e9a1da76abd0a756a3a9d775f5ed1>::operator()() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:4598]
UE4Editor_Slate!FEventRouter::Route<FReply,FEventRouter::FToLeafmostPolicy,FPointerEvent,<lambda_8d0e9a1da76abd0a756a3a9d775f5ed1> >() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:215]
UE4Editor_Slate!FSlateApplication::RoutePointerUpEvent() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:4587]
UE4Editor_Slate!FSlateApplication::ProcessMouseButtonUpEvent() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5041]
UE4Editor_Slate!FSlateApplication::OnMouseUp() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5021]
UE4Editor_Core!FWindowsApplication::ProcessDeferredMessage() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\private\windows\windowsapplication.cpp:1510]
UE4Editor_Core!FWindowsApplication::DeferMessage() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\private\windows\windowsapplication.cpp:1850]
UE4Editor_Core!FWindowsApplication::ProcessMessage() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\private\windows\windowsapplication.cpp:745]
UE4Editor_Core!FWindowsApplication::AppWndProc() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\private\windows\windowsapplication.cpp:667]
user32
user32
UE4Editor_Core!FWindowsPlatformMisc::PumpMessages() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\private\windows\windowsplatformmisc.cpp:903]
UE4Editor!FEngineLoop::Tick() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\launch\private\launchengineloop.cpp:2729]
UE4Editor!GuardedMain() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\launch\private\launch.cpp:148]
UE4Editor!GuardedMainWrapper() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:126]
UE4Editor!WinMain() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:200]
UE4Editor!__scrt_common_main_seh() [f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:264]
kernel32
ntdll