Crash when Reimporting SpeedTree asset with Billboard Material reset to the default World Grid Material

A crash occurs when reimporting a SpeedTree asset after clearing the material billboard element and setting it to the default world grid material.

The licensee reporting the issue mentions to workaround the crash is to have any material other than the default world grid material.

This crash also occurs the binary releases below:
4.13.2 - 3172292
4.14.3 - 3249277
4.15 Preview 4 - 3286799

1. Create new blank blueprint project.
2. Import attached SpeedTree asset (BroadLeaf_Desktop)
3. For the import settings, Set the Geometry to 'Both' and the LOD Setup to 'Individual Actors'.
4. Import and Save All.
5. Open the mesh and within the Static Mesh editor, reset Element 5 (Broadleaf_Desktop_BillboardMat) to the default World Grid Material.
6. Save the asset.
7. Right-click the static mesh within the Content Browser and choose 'Reimport'
8. Use same settings mentioned in step 3.
9. Click Import

Outcome A crash occurs after attempting to reimport the mesh.

Expected The mesh imports and resets the cleared material element to the correct billboard material.

UE4Editor_SpeedTreeImporter!USpeedTreeImportFactory::FactoryCreateBinary() speedtreeimportfactory.cpp:1225
UE4Editor_UnrealEd!UFactory::FactoryCreateBinary() factory.h:286
UE4Editor_UnrealEd!UFactory::FactoryCreateFile() factory.cpp:75
UE4Editor_UnrealEd!UFactory::ImportObject() factory.cpp:150
UE4Editor_SpeedTreeImporter!UReimportSpeedTreeFactory::Reimport() reimportspeedtreefactory.cpp:83
UE4Editor_UnrealEd!FReimportManager::Reimport() editor.cpp:319
UE4Editor_UnrealEd!FReimportManager::ReimportMultiple() editor.cpp:504
UE4Editor_UnrealEd!FReimportManager::ValidateAllSourceFileAndReimport() editor.cpp:487
UE4Editor_ContentBrowser!FAssetContextMenu::ExecuteReimport() assetcontextmenu.cpp:1355
UE4Editor_ContentBrowser!TBaseSPMethodDelegateInstance<0,FAssetContextMenu,0,TTypeWrapper<void> __cdecl() delegateinstancesimpl.h:318
UE4Editor_ContentBrowser!TBaseSPMethodDelegateInstance<0,FAssetContextMenu,0,void __cdecl() delegateinstancesimpl.h:424
UE4Editor_Slate!SMenuEntryBlock::OnClicked() smenuentryblock.cpp:1042
UE4Editor_Slate!SMenuEntryBlock::OnMenuItemButtonClicked() smenuentryblock.cpp:993
UE4Editor_Slate!TMemberFunctionCaller<SMenuEntryBlock,FReply() delegateinstanceinterface.h:161
UE4Editor_Slate!TTupleImpl<TIntegerSequence<unsigned int> >::ApplyAfter<TMemberFunctionCaller<SMenuEntryBlock,FReply() tuple.h:116
UE4Editor_Slate!TBaseSPMethodDelegateInstance<0,SMenuEntryBlock,0,FReply __cdecl() delegateinstancesimpl.h:318
UE4Editor_Slate!TBaseDelegate<FReply>::Execute() delegatesignatureimpl.inl:522
UE4Editor_Slate!SButton::OnMouseButtonUp() sbutton.cpp:275
UE4Editor_Slate!SMenuEntryButton::OnMouseButtonUp() smenuentryblock.cpp:391
UE4Editor_Slate!<lambda_8d0e9a1da76abd0a756a3a9d775f5ed1>::operator() slateapplication.cpp:5010
UE4Editor_Slate!FEventRouter::Route<FReply,FEventRouter::FToLeafmostPolicy,FPointerEvent,<lambda_8d0e9a1da76abd0a756a3a9d775f5ed1> >() slateapplication.cpp:215
UE4Editor_Slate!FSlateApplication::RoutePointerUpEvent() slateapplication.cpp:4999
UE4Editor_Slate!FSlateApplication::ProcessMouseButtonUpEvent() slateapplication.cpp:5462
UE4Editor_Slate!FSlateApplication::OnMouseUp() slateapplication.cpp:5439
UE4Editor_Core!FWindowsApplication::ProcessDeferredMessage() windowsapplication.cpp:1652
UE4Editor_Core!FWindowsApplication::DeferMessage() windowsapplication.cpp:2071
UE4Editor_Core!FWindowsApplication::ProcessMessage() windowsapplication.cpp:814
UE4Editor_Core!FWindowsApplication::AppWndProc() windowsapplication.cpp:678
user32!UserCallWinProcCheckWow()
user32!DispatchMessageWorker()
UE4Editor_Core!FWindowsPlatformMisc::PumpMessages() windowsplatformmisc.cpp:951
UE4Editor!FEngineLoop::Tick() launchengineloop.cpp:2811
UE4Editor!GuardedMain() launch.cpp:152
UE4Editor!GuardedMainWrapper() launchwindows.cpp:126
UE4Editor!WinMain() launchwindows.cpp:202
UE4Editor!__scrt_common_main_seh() exe_common.inl:264
kernel32!BaseThreadInitThunk()
ntdll!RtlUserThreadStart()