Handle fracturing a destructible mesh when the source mesh has been deleted without a crash.

When attempting to fracture a destructible where the source static mesh has been deleted we get a crash in APEX.

APEX can't seem to find any root leaf nodes to begin fracturing upon. This does not happen when the mesh is present.

To repro, open the project in the UDN ticket and then open any destructible mesh and attempt to fracture it.

We should more gracefully handle this situation (at least without a crash) or attempt to make the import from apex asset work in this case.

1) Open UDN project
2) Open cup destructible mesh
3) Click fracture

APEX_DestructiblePROFILE_x64 APEX_DestructiblePROFILE_x64 UE4Editor_Engine!UDestructibleFractureSettings::CreateVoronoiSitesInRootMesh() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\engine\private\destructiblefracturesettings.cpp:488] UE4Editor_DestructibleMeshEditor!FDestructibleMeshEditorViewportClient::Fracture() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\editor\destructiblemesheditor\private\sdestructiblemesheditorviewport.cpp:218] UE4Editor_DestructibleMeshEditor!TBaseSPMethodDelegateInstance<0,FDestructibleMeshEditorViewportClient,0,TTypeWrapper __cdecl(void)>::Execute() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:282] UE4Editor_DestructibleMeshEditor!TBaseSPMethodDelegateInstance<0,FDestructibleMeshEditorViewportClient,0,void __cdecl(void)>::ExecuteIfSafe() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:388] UE4Editor_Slate!FUICommandList::ExecuteAction() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\slate\private\framework\commands\uicommandlist.cpp:75] UE4Editor_Slate!SToolBarButtonBlock::OnClicked() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\slate\private\framework\multibox\stoolbarbuttonblock.cpp:296] UE4Editor_Slate!TMemberFunctionCaller::operator()<>() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\core\public\delegates\delegateinstanceinterface_variadics.h:161] UE4Editor_Slate!TTupleImpl >::ApplyAfter_ExplicitReturnType >() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\core\public\delegates\tuple.h:113] UE4Editor_Slate!TBaseSPMethodDelegateInstance<0,SToolBarButtonBlock,0,FReply __cdecl(void)>::Execute() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:282] UE4Editor_Slate!TBaseDelegate::Execute() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\core\public\delegates\delegatesignatureimpl_variadics.inl:440] UE4Editor_Slate!SButton::OnMouseButtonUp() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\slate\private\widgets\input\sbutton.cpp:225] UE4Editor_Slate!::operator()() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:4170] UE4Editor_Slate!FEventRouter::Route >() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:212] UE4Editor_Slate!FSlateApplication::ProcessMouseButtonUpEvent() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:4173] UE4Editor_Slate!FSlateApplication::OnMouseUp() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:4133] UE4Editor_Core!FWindowsApplication::ProcessDeferredMessage() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\core\private\windows\windowsapplication.cpp:1406] UE4Editor_Core!FWindowsApplication::DeferMessage() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\core\private\windows\windowsapplication.cpp:1711] UE4Editor_Core!FWindowsApplication::ProcessMessage() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\core\private\windows\windowsapplication.cpp:706] UE4Editor_Core!FWindowsApplication::AppWndProc() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\core\private\windows\windowsapplication.cpp:628] user32 user32 UE4Editor_Core!FWindowsPlatformMisc::PumpMessages() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\core\private\windows\windowsplatformmisc.cpp:792] UE4Editor!FEngineLoop::Tick() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\launch\private\launchengineloop.cpp:2323] UE4Editor!GuardedMain() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\launch\private\launch.cpp:142] UE4Editor!GuardedMainWrapper() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\launch\private\windows\launchwindows.cpp:126] UE4Editor!WinMain() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\source\runtime\launch\private\windows\launchwindows.cpp:200]