I see this warning in 4.14.0 when opening a project without a tutorial in it.
The warning is triggered in StaticLoadObject (in Runtime/CoreUObject/Private/UObject/UObjectGlobals.cpp, lines 872ff), called from a call to LoadClass in FIntroTutorials::MaybeOpenWelcomeTutorial() (in Source/Editor/IntroTutorials/Private/IntroTutorials.cpp, line 329), which is commented with “try project startup tutorial”.
The only meaningful parameter given is *GetDefault()->StartupTutorial.ToString(), which returns an empty string – not surprising, since there is no tutorial.
A fix might be as simple as checking this string for not being empty before calling LoadClass.
Or (since this warning sometimes seems to occur in other circumstances too) maybe LoadClass/LoadObject could even silently return null if the given name is empty? The caller should check for a valid result anyway (which is done here, so no further error occurs). Or change the warning to something like “name must be valid” or “empty object name given to LoadObject”?