Similar issue here. Tried nativizing only a few blueprints.
[2017.10.29-21.15.12:576][ 0]LogThreadingWindows: Error: Runnable thread FAsyncLoadingThread crashed.
[2017.10.29-21.15.12:576][ 0]LogWindows: Error: === Critical error: ===
[2017.10.29-21.15.12:576][ 0]LogWindows: Error:
[2017.10.29-21.15.12:576][ 0]LogWindows: Error: Fatal error: [File:D:\Build\++UE4+Release-4.17+Compile\Sync\Engine\Source\Runtime\CoreUObject\Private\Serialization\AsyncLoading.cpp] [Line: 2858]
[2017.10.29-21.15.12:576][ 0]LogWindows: Error: Missing import of (/Game/Assets/UI/Widgets/Spartan/Spartan): WidgetNames_30_B1D16C014E889E872DA71BAAC14F3BDA in Package /Game/Code/CustomHUD/ST_HUDLayout was not found, but the package exists.