Crash when using merge actor on static meshes that have been affected by simplygon

UE4Editor_MaterialUtilities!FMaterialUtilities::RemapUniqueMaterialIndices() materialutilities.cpp:1497 
UE4Editor_MeshUtilities!FMeshUtilities::MergeStaticMeshComponents() meshutilities.cpp:7242 
UE4Editor_MergeActors!FMeshMergingTool::RunMerge() meshmergingtool.cpp:114 
UE4Editor_MergeActors!SMergeActorsToolbar::OnMergeActorsClicked() smergeactorstoolbar.cpp:132 
UE4Editor_MergeActors!TMemberFunctionCaller<SMergeActorsToolbar,FReply() delegateinstanceinterface.h:161 
UE4Editor_MergeActors!TTupleImpl<TIntegerSequence<unsigned int> >::ApplyAfter<TMemberFunctionCaller<SMergeActorsToolbar,FReply() tuple.h:115 
UE4Editor_MergeActors!TBaseSPMethodDelegateInstance<0,SMergeActorsToolbar,0,FReply __cdecl() delegateinstancesimpl.h:317 
UE4Editor_Slate!TBaseDelegate<FReply>::Execute() delegatesignatureimpl.inl:521 
UE4Editor_Slate!SButton::OnMouseButtonUp() sbutton.cpp:278 
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>