This is occurring in both 4.18 and 4.19 release branches.
Result: Ensure occurs
UE4Editor_Landscape!ULandscapeComponent::GetLayerDebugColorKey() landscape.cpp:445 UE4Editor_Landscape!ULandscapeComponent::PostEditUndo() landscapeedit.cpp:420 UE4Editor_UnrealEd!FTransaction::Apply() editortransaction.cpp:506 UE4Editor_UnrealEd!UTransBuffer::Undo() editortransaction.cpp:891 UE4Editor_UnrealEd!UEditorEngine::Exec_Transaction() editorserver.cpp:5046 UE4Editor_UnrealEd!UEditorEngine::Exec() editorserver.cpp:5493 UE4Editor_UnrealEd!UUnrealEdEngine::Exec() unrealedsrv.cpp:672 UE4Editor_MainFrame!FMainFrameActionCallbacks::ExecuteExecCommand() mainframeactions.cpp:1014 UE4Editor_MainFrame!TBaseStaticDelegateInstance<void __cdecl() delegateinstancesimpl.h:1027 UE4Editor_Slate!FUICommandList::ConditionalProcessCommandBindings() uicommandlist.cpp:216 UE4Editor_Slate!FUICommandList::ProcessCommandBindings() uicommandlist.cpp:160 UE4Editor_MainFrame!FMainFrameActionCallbacks::OnUnhandledKeyDownEvent() mainframeactions.cpp:212 UE4Editor_MainFrame!UE4Tuple_Private::TTupleImpl<TIntegerSequence<unsigned int> >::ApplyAfter<FReply() tuple.h:497 UE4Editor_MainFrame!TBaseStaticDelegateInstance<FReply __cdecl() delegateinstancesimpl.h:958 UE4Editor_Slate!TBaseDelegate<FReply,FKeyEvent const & __ptr64>::Execute() delegatesignatureimpl.inl:537 UE4Editor_Slate!FSlateApplication::ProcessKeyDownEvent() slateapplication.cpp:4845 UE4Editor_Slate!FSlateApplication::OnKeyDown() slateapplication.cpp:4744 UE4Editor_ApplicationCore!FWindowsApplication::ProcessDeferredMessage() windowsapplication.cpp:1560 UE4Editor_ApplicationCore!FWindowsApplication::DeferMessage() windowsapplication.cpp:2171 UE4Editor_ApplicationCore!FWindowsApplication::ProcessMessage() windowsapplication.cpp:888 UE4Editor_ApplicationCore!FWindowsApplication::AppWndProc() windowsapplication.cpp:725 user32!<Unknown> user32!<Unknown> UE4Editor_ApplicationCore!FWindowsPlatformApplicationMisc::PumpMessages() windowsplatformapplicationmisc.cpp:129 UE4Editor!FEngineLoop::Tick() launchengineloop.cpp:3220 UE4Editor!GuardedMain() launch.cpp:166 UE4Editor!GuardedMainWrapper() launchwindows.cpp:134 UE4Editor!WinMain() launchwindows.cpp:210 UE4Editor!__scrt_common_main_seh() exe_common.inl:253 kernel32!<Unknown> ntdll!<Unknown>
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