Errors while compiling VXGI

[MAGtechnologies]

All seems to be going ok with tsting. One problem which i can not work out and is causing many problems is :

Replacing, adding material to a static mesh.

If i change Any material on any slot on the mesh, I always get a crash, and is really annoying.

Here is the crash in :

MachineId:CE2E1A81418B38ECDF553FA484EC6E30
EpicAccountId:7f5de8cece6e488bb1be756133f01eeb

Unknown exception - code 00000001 (first/second chance not available)

"Assertion failed: StaticMesh.Materials.IsValidIndex(SlotIndex) [File:D:\U448CDFSFVXGI\UnrealEngine-4.8p3_VXGI\Engine\Source\Editor\StaticMeshEditor\Private\Stat

KERNELBASE
UE4Editor_Core!FOutputDeviceWindowsError::Serialize() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\private\windows\windowsplatformoutputdevices.cpp:95]
UE4Editor_Core!FOutputDevice::Logf__VA() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\private\misc\outputdevice.cpp:144]
UE4Editor_Core!FDebug::AssertFailed() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\private\misc\outputdevice.cpp:360]
UE4Editor_StaticMeshEditor!FMeshSectionSettingsLayout::OnMaterialChanged() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\editor\staticmesheditor\private\staticmesheditortools.cpp:1259]
UE4Editor_StaticMeshEditor!TBaseSPMethodDelegateInstance<0,FMeshSectionSettingsLayout,0,TTypeWrapper<void> __cdecl(UMaterialInterface * __ptr64,UMaterialInterface * __ptr64,int,bool)>::Execute() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:282]
UE4Editor_StaticMeshEditor!TBaseSPMethodDelegateInstance<0,FMeshSectionSettingsLayout,0,void __cdecl(UMaterialInterface * __ptr64,UMaterialInterface * __ptr64,int,bool)>::ExecuteIfSafe() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:388]
UE4Editor_PropertyEditor!FMaterialItemView::ReplaceMaterial() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\editor\propertyeditor\private\propertycustomizationhelpers.cpp:763]
UE4Editor_PropertyEditor!TBaseSPMethodDelegateInstance<0,FMaterialItemView,0,TTypeWrapper<void> __cdecl(FAssetData const & __ptr64)>::Execute() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:282]
UE4Editor_PropertyEditor!TBaseSPMethodDelegateInstance<0,FMaterialItemView,0,void __cdecl(FAssetData const & __ptr64)>::ExecuteIfSafe() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:388]
UE4Editor_PropertyEditor!SPropertyEditorAsset::SetValue() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\editor\propertyeditor\private\userinterface\propertyeditor\spropertyeditorasset.cpp:525]
UE4Editor_PropertyEditor!SPropertyEditorAsset::OnUse() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\editor\propertyeditor\private\userinterface\propertyeditor\spropertyeditorasset.cpp:736]
UE4Editor_PropertyEditor!TBaseSPMethodDelegateInstance<0,SPropertyEditorAsset,0,TTypeWrapper<void> __cdecl(void)>::Execute() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:282]
UE4Editor_PropertyEditor!TBaseSPMethodDelegateInstance<0,SPropertyEditorAsset,0,void __cdecl(void)>::ExecuteIfSafe() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:388]
UE4Editor_PropertyEditor!PropertyCustomizationHelpers::SPropertyEditorButton::OnClick() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\editor\propertyeditor\private\propertycustomizationhelpers.cpp:67]
UE4Editor_PropertyEditor!TMemberFunctionCaller<PropertyCustomizationHelpers::SPropertyEditorButton,FReply (__cdecl PropertyCustomizationHelpers::SPropertyEditorButton::)(void) __ptr64>::operator()<>() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\public\delegates\delegateinstanceinterface_variadics.h:161]
UE4Editor_PropertyEditor!TTupleImpl<TIntegerSequence<> >::ApplyAfter_ExplicitReturnType<FReply,TMemberFunctionCaller<PropertyCustomizationHelpers::SPropertyEditorButton,FReply (__cdecl PropertyCustomizationHelpers::SPropertyEditorButton::)(void) __ptr64> >() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\public\delegates uple.h:113]
UE4Editor_PropertyEditor!TBaseSPMethodDelegateInstance<0,PropertyCustomizationHelpers::SPropertyEditorButton,0,FReply __cdecl(void)>::Execute() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:282]
UE4Editor_Slate!TBaseDelegate<FReply>::Execute() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\public\delegates\delegatesignatureimpl_variadics.inl:440]
UE4Editor_Slate!SButton::OnMouseButtonUp() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\slate\private\widgets\input\sbutton.cpp:225]
UE4Editor_Slate!<lambda_de96dc3471181973108233c6db1f9843>::operator()() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:4170]
UE4Editor_Slate!FEventRouter::Route<FReply,FEventRouter::FToLeafmostPolicy,FPointerEvent,<lambda_de96dc3471181973108233c6db1f9843> >() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:212]
UE4Editor_Slate!FSlateApplication::ProcessMouseButtonUpEvent() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:4173]
UE4Editor_Slate!FSlateApplication::OnMouseUp() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:4133]
UE4Editor_Core!FWindowsApplication::ProcessDeferredMessage() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\private\windows\windowsapplication.cpp:1406]
UE4Editor_Core!FWindowsApplication::DeferMessage() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\private\windows\windowsapplication.cpp:1711]
UE4Editor_Core!FWindowsApplication::ProcessMessage() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\private\windows\windowsapplication.cpp:706]
UE4Editor_Core!FWindowsApplication::AppWndProc() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\private\windows\windowsapplication.cpp:628]
user32
user32
UE4Editor_Core!FWindowsPlatformMisc::PumpMessages() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\core\private\windows\windowsplatformmisc.cpp:792]
UE4Editor!FEngineLoop::Tick() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\launch\private\launchengineloop.cpp:2325]
UE4Editor!GuardedMain() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\launch\private\launch.cpp:142]
UE4Editor!GuardedMainWrapper() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\launch\private\windows\launchwindows.cpp:126]
UE4Editor!WinMain() [d:\u448cdfsfvxgi\unrealengine-4.8p3_vxgi\engine\source\runtime\launch\private\windows\launchwindows.cpp:200]

This happens on any mesh i try to change any material on. Any advice would be great