Using v11.0.2-012, this error is immediately thrown every time I log in:

18x ...aceProfessionShoppingList/modules/Tweaks.lua:261: attempt to index local 'f' (a nil value)
[string "@ProfessionShoppingList/modules/Tweaks.lua"]:261: in function DisableHandyNotesAltRMB' [string "@ProfessionShoppingList/modules/Tweaks.lua"]:35: in function ?'
[string "@ProfessionShoppingList/modules/Tweaks.lua"]:17: in function <...aceProfessionShoppingList/modules/Tweaks.lua:15>

Locals:
f = nil
f2 = Frame {
}
(*temporary) = "Frame"
(*temporary) = nil
(*temporary) = nil
(*temporary) = "attempt to index local 'f' (a nil value)"

I'm unable to reproduce this error on my end.
Does the error occur when PSL and HandyNotes (and BugSack + BugGrabber, if you use them) are the only addons enabled? I suspect another addon has an outdated library that's interfering.