I don't know why it failed to find dependency index for object property

UATHelper: Packaging (Windows (64-bit)): UE4Editor-Cmd: [2017.06.11-23.17.24:207][ 0]LogWindows:Error: Assertion failed: !DependencyIndex.IsNull() [File:D:\Build++UE4+Release-4.13+Compile\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\SavePackage.cpp] [Line: 4366]
UATHelper: Packaging (Windows (64-bit)): UE4Editor-Cmd: [2017.06.11-23.17.24:207][ 0]LogWindows:Error: Failed to find dependency index for ObjectProperty /Engine/VREditor/Tutorial/UMG/Buttons/SlideButtons.SlideButtons_C:PreviousButton (Function /Engine/VREditor/Tutorial/UMG/Slides/Option01/Option_01.Option_01_C:CheckForButtonEnabled)
UATHelper: Packaging (Windows (64-bit)): UE4Editor-Cmd: [2017.06.11-23.17.24:209][ 0]LogWindows:Error:
UATHelper: Packaging (Windows (64-bit)): UE4Editor-Cmd: [2017.06.11-23.17.24:209][ 0]LogWindows:Error:
UATHelper: Packaging (Windows (64-bit)): UE4Editor-Cmd: [2017.06.11-23.17.24:209][ 0]LogWindows:Error:
UATHelper: Packaging (Windows (64-bit)): UE4Editor-Cmd: [2017.06.11-23.17.24:209][ 0]LogWindows:Error: end: stack for UAT
UATHelper: Packaging (Windows (64-bit)): CommandUtils.Run: Run: Took 129.7208001s to run UE4Editor-Cmd.exe, ExitCode=3