Crash when using Get Start/End Frame (Seconds) node with MovieSceneSection have no opened bound

If use Get Start/End Frame (Seconds) with GetStartFrame has a section isBounded is False, eg 3DTransformSection, editor will be crashed.

Specifically, a following code occur crash.

inline FFrameNumber DiscreteInclusiveLower(const TRangeBound<FFrameNumber>& InLowerBound)
{
   check(!InLowerBound.IsOpen());

Probably, we should sepate processing by open or close, or output a warning instead of crash

1. Open repro proj
2. Run "NewEditorUtilityWidgetBlueprint" and click button
3. crash

UE4Editor_Core!FWindowsErrorOutputDevice::Serialize() [d:\build\++ue4\sync\engine\source\runtime\core\private\windows\windowserroroutputdevice.cpp:79]
UE4Editor_Core!FOutputDevice::LogfImpl() [d:\build\++ue4\sync\engine\source\runtime\core\private\misc\outputdevice.cpp:71]
UE4Editor_Core!AssertFailedImplV() [d:\build\++ue4\sync\engine\source\runtime\core\private\misc\assertionmacros.cpp:101]
UE4Editor_Core!FDebug::CheckVerifyFailedImpl() [d:\build\++ue4\sync\engine\source\runtime\core\private\misc\assertionmacros.cpp:439]
UE4Editor_SequencerScripting!UMovieSceneSectionExtensions::GetStartFrame() [d:\build\++ue4\sync\engine\plugins\moviescene\sequencerscripting\source\sequencerscripting\private\extensionlibraries\moviescenesectionextensions.cpp:33]
UE4Editor_SequencerScripting!UMovieSceneSectionExtensions::execGetStartFrame() [d:\build\++ue4\sync\engine\plugins\moviescene\sequencerscripting\source\sequencerscripting\public\extensionlibraries\moviescenesectionextensions.h:21]
UE4Editor_CoreUObject!UFunction::Invoke() [d:\build\++ue4\sync\engine\source\runtime\coreuobject\private\uobject\class.cpp:4643]
UE4Editor_CoreUObject!UObject::CallFunction() [d:\build\++ue4\sync\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:904]
UE4Editor_CoreUObject!UObject::ProcessContextOpcode() [d:\build\++ue4\sync\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:2314]
UE4Editor_CoreUObject!UObject::execLet() [d:\build\++ue4\sync\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:2100]
UE4Editor_CoreUObject!ProcessLocalScriptFunction() [d:\build\++ue4\sync\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:974]
UE4Editor_CoreUObject!ProcessScriptFunction<void (__cdecl*)(UObject *,FFrame &,void *)>() [d:\build\++ue4\sync\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:809]
UE4Editor_CoreUObject!ProcessLocalFunction() [d:\build\++ue4\sync\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:1033]
UE4Editor_CoreUObject!ProcessLocalScriptFunction() [d:\build\++ue4\sync\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:974]
UE4Editor_CoreUObject!UObject::ProcessInternal() [d:\build\++ue4\sync\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:1058]
UE4Editor_CoreUObject!UFunction::Invoke() [d:\build\++ue4\sync\engine\source\runtime\coreuobject\private\uobject\class.cpp:4643]
UE4Editor_CoreUObject!UObject::ProcessEvent() [d:\build\++ue4\sync\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:1464]
UE4Editor_UMG!TMulticastScriptDelegate<FWeakObjectPtr>::ProcessMulticastDelegate<UObject>() [d:\build\++ue4\sync\engine\source\runtime\core\public\uobject\scriptdelegates.h:488]
UE4Editor_UMG!TBaseUObjectMethodDelegateInstance<0,UButton,void __cdecl(void)>::ExecuteIfSafe() [d:\build\++ue4\sync\engine\source\runtime\core\public\delegates\delegateinstancesimpl.h:679]
UE4Editor_Slate!SButton::OnMouseButtonDown() [d:\build\++ue4\sync\engine\source\runtime\slate\private\widgets\input\sbutton.cpp:218]
UE4Editor_Slate!FEventRouter::Route<FReply,FEventRouter::FBubblePolicy,FPointerEvent,<lambda_6264868de66784e8427f79716494ab5b> >() [d:\build\++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:271]
UE4Editor_Slate!FSlateApplication::RoutePointerDownEvent() [d:\build\++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5487]
UE4Editor_Slate!FSlateApplication::ProcessMouseButtonDownEvent() [d:\build\++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5440]
UE4Editor_Slate!FSlateApplication::OnMouseDown() [d:\build\++ue4\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5353]
UE4Editor_ApplicationCore!FWindowsApplication::ProcessDeferredMessage() [d:\build\++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:1850]
UE4Editor_ApplicationCore!FWindowsApplication::DeferMessage() [d:\build\++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:2292]
UE4Editor_ApplicationCore!FWindowsApplication::ProcessMessage() [d:\build\++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:1522]
UE4Editor_ApplicationCore!FWindowsApplication::AppWndProc() [d:\build\++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsapplication.cpp:772]
user32
user32
UE4Editor_ApplicationCore!FWindowsPlatformApplicationMisc::PumpMessages() [d:\build\++ue4\sync\engine\source\runtime\applicationcore\private\windows\windowsplatformapplicationmisc.cpp:130]
UE4Editor!FEngineLoop::Tick() [d:\build\++ue4\sync\engine\source\runtime\launch\private\launchengineloop.cpp:3882]
UE4Editor!GuardedMain() [d:\build\++ue4\sync\engine\source\runtime\launch\private\launch.cpp:168]
UE4Editor!GuardedMainWrapper() [d:\build\++ue4\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:145]
UE4Editor!WinMain() [d:\build\++ue4\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:275]
UE4Editor!__scrt_common_main_seh() [d:\agent\_work\3\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288]
kernel32
ntdll