Well, the backtrace tells us it is on line 39, so it must be in:
Lua Code:
Dek01StaminaBarWindow:SetHidden(not Dek01.savedVars.Show)
I'd guess that either the window wasn't created, possibly due to forgetting to add the XML to the manifest, or that your defaults / saved vars don't have the show value.
You can investigate using the `d` function in there, to dump out the various things you are looking at.