Not resolved in 4.14 as I am having exact same issue:
Assertion failed: !DependencyIndex.IsNull() [File:D:\Build++UE4+Release-4.14+Compile\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage.cpp] [Line: 4600]
UATHelper: Packaging (Windows (64-bit)): UE4Editor-Cmd: [2016.11.30-00.34.20:252][ 0]LogWindows:Error: Failed to find dependency index for WidgetBlueprint /Game/BluePrints/UI/videoLibrary_exp.videoLibrary_exp (MotionControllerMap_C /Game/VirtualRealityBP/Maps/MotionControllerMap.Default__MotionControllerMap_C)
what is the fix for it? I’ve removed that actor from all levels (kept source file) but still same thing is happening while cooking / building the project.