[4.6.1]crash when saving material in mac

hi, i follow the tutorial here: - YouTube

but when i try to save the material, it crash, and give me the errors:
Fatal error: [File:/Users/apple/workspace/UnrealEngine/Engine/Source/Runtime/CoreUObject/Private/UObject/LinkerSave.cpp] [Line: 24]
Error opening file ‘…/…/…/…/…/Documents/Unreal Projects/TutorialCode/Saved/CharacterMaterial6210FCAD5E4A0D2E668EA78BB0C99EC1.tmp’.

any one know what happen?