Crash undoing in Montage editor with no montage sections

Crash occur in the animation montage editor when the user does an undo action with no montage sections.

1. Open Content Examples in editor.
2. Create a new montage based on Owen_Skeleton.
3. Open it in editor.
4. Press Ctrl+Z.

RESULT

Crash.

Assertion failed: (Index >= 0) & (Index < ArrayNum) [Link Removed] [Line: 637]
Array index out of bounds: -1 from an array of size 0

KERNELBASE.dll!0x00000000FDA0A06D
UE4Editor-Core.dll!FOutputDeviceWindowsError::Serialize() [d:\animphys\engine\source\runtime\core\private\windows\windowsplatformoutputdevices.cpp:120]
UE4Editor-Core.dll!FOutputDevice::Logf__VA() [d:\animphys\engine\source\runtime\core\private\misc\outputdevice.cpp:70]
UE4Editor-Core.dll!FDebug::AssertFailed() [d:\animphys\engine\source\runtime\core\private\misc\assertionmacros.cpp:349]
UE4Editor-Persona.dll!SMontageEditor::OnSetMontagePreviewSlot() [d:\animphys\engine\source\editor\persona\private\smontageeditor.cpp:291]
UE4Editor-Persona.dll!TBaseSPMethodDelegateInstance<0,SMontageEditor,0,TTypeWrapper<void> __cdecl(int)>::Execute() [d:\animphys\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:327]
UE4Editor-Persona.dll!TBaseSPMethodDelegateInstance<0,SMontageEditor,0,void __cdecl(int)>::ExecuteIfSafe() [d:\animphys\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:434]
UE4Editor-Persona.dll!SAnimMontagePanel::Update() [d:\animphys\engine\source\editor\persona\private\sanimmontagepanel.cpp:80]
UE4Editor-Persona.dll!SMontageEditor::SortAndUpdateMontage() [d:\animphys\engine\source\editor\persona\private\smontageeditor.cpp:604]
UE4Editor-Persona.dll!SMontageEditor::RebuildMontagePanel() [d:\animphys\engine\source\editor\persona\private\smontageeditor.cpp:516]
UE4Editor-Persona.dll!TBaseSPMethodDelegateInstance<0,SMontageEditor,0,TTypeWrapper<void> __cdecl(void)>::Execute() [d:\animphys\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:327]
UE4Editor-Persona.dll!TBaseSPMethodDelegateInstance<0,SMontageEditor,0,void __cdecl(void)>::ExecuteIfSafe() [d:\animphys\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:434]
UE4Editor-AnimationEditor.dll!TBaseMulticastDelegate<void>::Broadcast() [d:\animphys\engine\source\runtime\core\public\delegates\delegatesignatureimpl.inl:937]
UE4Editor-UnrealEd.dll!UEditorEngine::BroadcastPostUndo() [d:\animphys\engine\source\editor\unrealed\private\editorserver.cpp:5122]
UE4Editor-UnrealEd.dll!UEditorEngine::HandleTransactorUndo() [d:\animphys\engine\source\editor\unrealed\private\editorserver.cpp:1221]
UE4Editor-UnrealEd.dll!TBaseUObjectMethodDelegateInstance<0,UEditorEngine,TTypeWrapper<void> __cdecl(FUndoSessionContext,bool)>::Execute() [d:\animphys\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:766]
UE4Editor-UnrealEd.dll!TBaseUObjectMethodDelegateInstance<0,UEditorEngine,void __cdecl(FUndoSessionContext,bool)>::ExecuteIfSafe() [d:\animphys\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:856]
UE4Editor-UnrealEd.dll!TBaseMulticastDelegate<void,FUndoSessionContext,bool>::Broadcast() [d:\animphys\engine\source\runtime\core\public\delegates\delegatesignatureimpl.inl:937]
UE4Editor-UnrealEd.dll!UTransBuffer::Undo() [d:\animphys\engine\source\editor\unrealed\private\editortransaction.cpp:866]
UE4Editor-UnrealEd.dll!UEditorEngine::Exec_Transaction() [d:\animphys\engine\source\editor\unrealed\private\editorserver.cpp:5117]
UE4Editor-UnrealEd.dll!UEditorEngine::Exec() [d:\animphys\engine\source\editor\unrealed\private\editorserver.cpp:5564]
UE4Editor-UnrealEd.dll!UUnrealEdEngine::Exec() [d:\animphys\engine\source\editor\unrealed\private\unrealedsrv.cpp:672]
UE4Editor-MainFrame.dll!FMainFrameActionCallbacks::ExecuteExecCommand() [d:\animphys\engine\source\editor\mainframe\private\frame\mainframeactions.cpp:973]
UE4Editor-MainFrame.dll!TBaseStaticDelegateInstance<void __cdecl(void),FString>::ExecuteIfSafe() [d:\animphys\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:1027]
UE4Editor-Slate.dll!FUICommandList::ConditionalProcessCommandBindings() [d:\animphys\engine\source\runtime\slate\private\framework\commands\uicommandlist.cpp:216]
UE4Editor-Slate.dll!FUICommandList::ProcessCommandBindings() [d:\animphys\engine\source\runtime\slate\private\framework\commands\uicommandlist.cpp:160]
UE4Editor-MainFrame.dll!FMainFrameActionCallbacks::OnUnhandledKeyDownEvent() [d:\animphys\engine\source\editor\mainframe\private\frame\mainframeactions.cpp:211]
UE4Editor-MainFrame.dll!UE4Tuple_Private::TTupleImpl<TIntegerSequence<unsigned int> >::ApplyAfter<FReply (__cdecl*const & __ptr64)(FKeyEvent const & __ptr64),FKeyEvent const & __ptr64>() [d:\animphys\engine\source\runtime\core\public\templates\tuple.h:497]
UE4Editor-MainFrame.dll!TBaseStaticDelegateInstance<FReply __cdecl(FKeyEvent const & __ptr64)>::Execute() [d:\animphys\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:958]
UE4Editor-Slate.dll!TBaseDelegate<FReply,FKeyEvent const & __ptr64>::Execute() [d:\animphys\engine\source\runtime\core\public\delegates\delegatesignatureimpl.inl:537]
UE4Editor-Slate.dll!FSlateApplication::ProcessKeyDownEvent() [d:\animphys\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:4674]
UE4Editor-Slate.dll!FSlateApplication::OnKeyDown() [d:\animphys\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:4574]
UE4Editor-Core.dll!FWindowsApplication::ProcessDeferredMessage() [d:\animphys\engine\source\runtime\core\private\windows\windowsapplication.cpp:1539]
UE4Editor-Core.dll!FWindowsApplication::DeferMessage() [d:\animphys\engine\source\runtime\core\private\windows\windowsapplication.cpp:2127]
UE4Editor-Core.dll!FWindowsApplication::ProcessMessage() [d:\animphys\engine\source\runtime\core\private\windows\windowsapplication.cpp:867]
UE4Editor-Core.dll!FWindowsApplication::AppWndProc() [d:\animphys\engine\source\runtime\core\private\windows\windowsapplication.cpp:714]
USER32.dll!0x0000000077919BBD
USER32.dll!0x00000000779198C2
UE4Editor-Core.dll!FWindowsPlatformMisc::PumpMessages() [d:\animphys\engine\source\runtime\core\private\windows\windowsplatformmisc.cpp:1009]
UE4Editor.exe!FEngineLoop::Tick() [d:\animphys\engine\source\runtime\launch\private\launchengineloop.cpp:3077]
UE4Editor.exe!GuardedMain() [d:\animphys\engine\source\runtime\launch\private\launch.cpp:166]
UE4Editor.exe!GuardedMainWrapper() [d:\animphys\engine\source\runtime\launch\private\windows\launchwindows.cpp:134]
UE4Editor.exe!WinMain() [d:\animphys\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!0x0000000077A159CD
ntdll.dll!0x0000000077B4A561
ntdll.dll!0x0000000077B4A561