It doesn't happen every time, but it seems that sometimes Noesis ends up trying to operate on a viewport that isn't there.

This is what the callstack says:

TSharedPtr::ToSharedRef() SharedPointer.h:1046
UNoesisInstance::NativeOnMouseMove(const FGeometry &, const FPointerEvent &) NoesisInstance.cpp:1919
SObjectWidget::OnMouseMove(const FGeometry &, const FPointerEvent &) SObjectWidget.cpp:313
FSlateApplication::RoutePointerMoveEvent'::57'::<lambda_6>::operator()(const FArrangedWidget &,const FPointerEvent &) SlateApplication.cpp:5870
FEventRouter::Route<FReply,FEventRouter::FBubblePolicy,FPointerEvent,FSlateApplication::RoutePointerMoveEvent'::57'::<lambda_6> >(FSlateApplication ,FBubblePolicy,FPointerEvent,const <lambda_6> &,ESlateDebuggingInputEvent) SlateApplication.cpp:459
FSlateApplication::RoutePointerMoveEvent(const FWidgetPath &, const FPointerEvent &, bool) SlateApplication.cpp:5827
FSlateApplication::ProcessMouseMoveEvent(const FPointerEvent &, bool) SlateApplication.cpp:6308
FSlateApplication::OnMouseMove() SlateApplication.cpp:6229
FWindowsApplication::ProcessDeferredMessage(const FDeferredWindowsMessage &) WindowsApplication.cpp:3463
FWindowsApplication::DeferMessage(TSharedPtr<…> &, HWND__ , unsigned int, unsigned long long, long long, int, int, unsigned int) WindowsApplication.cpp:4120
FWindowsApplication::ProcessMessage(HWND , unsigned int, unsigned long long, long long) WindowsApplication.cpp:2355
[Inlined] WindowsApplication_WndProc(HWND__ , unsigned int, unsigned long long, long long) WindowsApplication.cpp:2142
FWindowsApplication::AppWndProc(HWND__ , unsigned int, unsigned long long, long long) WindowsApplication.cpp:2147
[Inlined] WinPumpMessages() WindowsPlatformApplicationMisc.cpp:124
FWindowsPlatformApplicationMisc::PumpMessages(bool) WindowsPlatformApplicationMisc.cpp:153
FEngineLoop::Tick() LaunchEngineLoop.cpp:5768
[Inlined] EngineTick() Launch.cpp:60
GuardedMain(const wchar_t ) Launch.cpp:192
LaunchWindowsStartup(HINSTANCE , HINSTANCE__ , char , int, const wchar_t ) LaunchWindows.cpp:266
WinMain(HINSTANCE *, HINSTANCE , char , int) LaunchWindows.cpp:334