Merge Actors with assigned same material to multiple slots causes an assert

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I attached a project for it.

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1. Make a static mesh with 2 or more material slots
2. Set same material to them
3. Place it to a level
4. Select "Merge actors" on the RMB menu.
5. Select "Harvest geometry ~" tab on the top of the dialog
6. Press "Merge Actors" on the bottom
7. An assert dialog is shown up.

This is a project that contains a more complex scene created by the TANGO.
This zip file is over 500MB, I stored it in the google drive.
Please download it with following link

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1. Open the project
2. Open "Test" level
3. Merge actors on levels in the above way

> UE4Editor-MeshMergeUtilities.dll!FMeshMergeUtilities::CreateProxyMesh::__l2::<lambda>(TArray<FFlattenMaterial,FDefaultAllocator> & FlattenedMaterialArray) Line 1472 C++
[Inline Frame] UE4Editor-ProxyLODMeshReduction.dll!TBaseDelegate<TTypeWrapper<void>,TArray<FFlattenMaterial,FDefaultAllocator> &>::Execute(TArray<FFlattenMaterial,FDefaultAllocator> &) Line 561 C++
UE4Editor-ProxyLODMeshReduction.dll!FVoxelizeMeshMerging::ProxyLOD::__l46::<lambda>() Line 842 C++
UE4Editor-ProxyLODMeshReduction.dll!tbb::internal::task_group_base::internal_run_and_wait<void <lambda>(void) const >(const FVoxelizeMeshMerging::ProxyLOD::__l46::void <lambda>(void) & f) Line 91 C++
[Inline Frame] UE4Editor-ProxyLODMeshReduction.dll!tbb::task_group::run_and_wait(const FVoxelizeMeshMerging::ProxyLOD::__l46::void <lambda>(void) &) Line 184 C++
[Inline Frame] UE4Editor-ProxyLODMeshReduction.dll!ProxyLOD::FTaskGroup::RunAndWait(const FVoxelizeMeshMerging::ProxyLOD::__l46::void <lambda>(void) &) Line 214 C++
UE4Editor-ProxyLODMeshReduction.dll!FVoxelizeMeshMerging::ProxyLOD(const TArray<FMeshMergeData,FDefaultAllocator> & InData, const FMeshProxySettings & InProxySettings, const TArray<FFlattenMaterial,FDefaultAllocator> & InputMaterials, const FGuid InJobGUID) Line 836 C++
UE4Editor-MeshMergeUtilities.dll!FMeshMergeUtilities::CreateProxyMesh(const TArray<UStaticMeshComponent *,FDefaultAllocator> & InComponentsToMerge, const FMeshProxySettings & InMeshProxySettings, UMaterialInterface * InBaseMaterial, UPackage * InOuter, const FString & InProxyBasePackageName, const FGuid InGuid, const TBaseDelegate<void,FGuid const ,TArray<UObject *,FDefaultAllocator> &> & InProxyCreatedDelegate, const bool bAllowAsync, const float ScreenSize) Line 1581 C++
UE4Editor-MeshMergeUtilities.dll!FMeshMergeUtilities::CreateProxyMesh(const TArray<UStaticMeshComponent *,FDefaultAllocator> & InStaticMeshComps, const FMeshProxySettings & InMeshProxySettings, UPackage * InOuter, const FString & InProxyBasePackageName, const FGuid InGuid, const TBaseDelegate<void,FGuid const ,TArray<UObject *,FDefaultAllocator> &> & InProxyCreatedDelegate, const bool bAllowAsync, const float ScreenSize) Line 1071 C++
UE4Editor-MergeActors.dll!FMeshProxyTool::RunMerge(const FString & PackageName) Line 153 C++
UE4Editor-MergeActors.dll!SMergeActorsToolbar::OnMergeActorsClicked() Line 148 C++
[Inline Frame] UE4Editor-MergeActors.dll!TMemberFunctionCaller<SMergeActorsToolbar,FReply (__cdecl SMergeActorsToolbar::*)(void)>::operator()() Line 156 C++
[Inline Frame] UE4Editor-MergeActors.dll!UE4Tuple_Private::TTupleImpl<TIntegerSequence<unsigned int> >::ApplyAfter(TMemberFunctionCaller<SMergeActorsToolbar,FReply (__cdecl SMergeActorsToolbar::*)(void)> &&) Line 498 C++
UE4Editor-MergeActors.dll!TBaseSPMethodDelegateInstance<0,SMergeActorsToolbar,0,FReply __cdecl(void)>::Execute() Line 279 C++
[Inline Frame] UE4Editor-Slate.dll!TBaseDelegate<FReply>::Execute() Line 561 C++
UE4Editor-Slate.dll!SButton::OnMouseButtonUp(const FGeometry & MyGeometry, const FPointerEvent & MouseEvent) Line 293 C++
[Inline Frame] UE4Editor-Slate.dll!FSlateApplication::RoutePointerUpEvent::__l8::<lambda_6c1f6b862bcfb82ea6dcab28fbcc9463>::operator()(const FArrangedWidget &) Line 5612 C++
UE4Editor-Slate.dll!FEventRouter::Route<FReply,FEventRouter::FToLeafmostPolicy,FPointerEvent,<lambda_6c1f6b862bcfb82ea6dcab28fbcc9463> >(FSlateApplication * ThisApplication, FEventRouter::FToLeafmostPolicy RoutingPolicy, FPointerEvent EventCopy, const FSlateApplication::RoutePointerUpEvent::__l8::<lambda_6c1f6b862bcfb82ea6dcab28fbcc9463> & Lambda) Line 271 C++
UE4Editor-Slate.dll!FSlateApplication::RoutePointerUpEvent(const FWidgetPath & WidgetsUnderPointer, const FPointerEvent & PointerEvent) Line 5598 C++
UE4Editor-Slate.dll!FSlateApplication::ProcessMouseButtonUpEvent(const FPointerEvent & MouseEvent) Line 6203 C++
UE4Editor-Slate.dll!FSlateApplication::OnMouseUp(const EMouseButtons::Type Button, const FVector2D CursorPos) Line 6176 C++
UE4Editor-ApplicationCore.dll!FWindowsApplication::ProcessDeferredMessage(const FDeferredWindowsMessage & DeferredMessage) Line 1842 C++
UE4Editor-ApplicationCore.dll!FWindowsApplication::DeferMessage(TSharedPtr<FWindowsWindow,0> & NativeWindow, HWND__ * InHWnd, unsigned int InMessage, unsigned __int64 InWParam, __int64 InLParam, int MouseX, int MouseY, unsigned int RawInputFlags) Line 2292 C++
UE4Editor-ApplicationCore.dll!FWindowsApplication::ProcessMessage(HWND__ * hwnd, unsigned int msg, unsigned __int64 wParam, __int64 lParam) Line 1522 C++
UE4Editor-ApplicationCore.dll!FWindowsApplication::AppWndProc(HWND__ * hwnd, unsigned int msg, unsigned __int64 wParam, __int64 lParam) Line 772 C++