Crash when Toggling Landscape Mode with Hidden Sub-Level containing a Landscape

A crash occurs when you have a hidden Sub-level containing a landscape and you attempt to toggle the Landscape Mode after re-opening the project.

The licensee has authored their own fix for this issue, and wanted to get insight to make sure they are implementing the fix correctly.

Regression?
Yes, I tested this in 4.12.5 and the editor does not crash.

1. Download and Open attached project.
2. Go to the Modes tab and select the Landscape mode.

Alternative Repro
1. Have a level (Main)
2. Have another level (Sub) with a landscape in it
3. Add the Sub level as a sub level to the Main level
4. Hide the Sub level. Save it all and close the editor. Reopen the editor and open the Main level.
5. Enable landscape edit mode

Outcome Editor crashes

Expected No crash occurs.

UE4Editor_Landscape!ALandscapeProxy::LandscapeActorToWorld() landscape.cpp:1571
UE4Editor_Landscape!ULandscapeInfo::GetLandscapeCenterPos() landscapeedit.cpp:2996
UE4Editor_Landscape!ALandscapeGizmoActiveActor::SetTargetLandscape() landscapegizmoactor.cpp:652
UE4Editor_LandscapeEditor!FEdModeLandscape::Enter() landscapeedmode.cpp:345
UE4Editor_UnrealEd!FEditorModeTools::ActivateMode() editormodemanager.cpp:386
UE4Editor_LevelEditor!SLevelEditor::ToggleEditorMode() sleveleditor.cpp:1223
UE4Editor_LevelEditor!TBaseStaticDelegateInstance<void __cdecl() delegateinstancesimpl.h:1017
UE4Editor_Slate!FUICommandList::ExecuteAction() uicommandlist.cpp:92
UE4Editor_Slate!SToolBarButtonBlock::OnClicked() stoolbarbuttonblock.cpp:300
UE4Editor_Slate!SToolBarButtonBlock::OnCheckStateChanged() stoolbarbuttonblock.cpp:326
UE4Editor_Slate!TBaseSPMethodDelegateInstance<0,SToolBarButtonBlock,0,TTypeWrapper<void> __cdecl() delegateinstancesimpl.h:317
UE4Editor_Slate!TBaseSPMethodDelegateInstance<0,SToolBarButtonBlock,0,void __cdecl() delegateinstancesimpl.h:424
UE4Editor_Slate!SCheckBox::ToggleCheckedState() scheckbox.cpp:293
UE4Editor_Slate!SCheckBox::OnMouseButtonUp() scheckbox.cpp:184
UE4Editor_Slate!<lambda_8d0e9a1da76abd0a756a3a9d775f5ed1>::operator() slateapplication.cpp:4901
UE4Editor_Slate!FEventRouter::Route<FReply,FEventRouter::FToLeafmostPolicy,FPointerEvent,<lambda_8d0e9a1da76abd0a756a3a9d775f5ed1> >() slateapplication.cpp:215
UE4Editor_Slate!FSlateApplication::RoutePointerUpEvent() slateapplication.cpp:4890
UE4Editor_Slate!FSlateApplication::ProcessMouseButtonUpEvent() slateapplication.cpp:5348
UE4Editor_Slate!FSlateApplication::OnMouseUp() slateapplication.cpp:5328
UE4Editor_Core!FWindowsApplication::ProcessDeferredMessage() windowsapplication.cpp:1584
UE4Editor_Core!FWindowsApplication::DeferMessage() windowsapplication.cpp:1930
UE4Editor_Core!FWindowsApplication::ProcessMessage() windowsapplication.cpp:747
UE4Editor_Core!FWindowsApplication::AppWndProc() windowsapplication.cpp:669
user32!<Unknown>
user32!<Unknown>
UE4Editor_Core!FWindowsPlatformMisc::PumpMessages() windowsplatformmisc.cpp:905
UE4Editor!FEngineLoop::Tick() launchengineloop.cpp:2788
UE4Editor!GuardedMain() launch.cpp:156
UE4Editor!GuardedMainWrapper() launchwindows.cpp:126
UE4Editor!WinMain() launchwindows.cpp:202
UE4Editor!__scrt_common_main_seh() exe_common.inl:264
kernel32!<Unknown>
ntdll!<Unknown>