[MunkDev]

function frame:m4xEventHandler(self, event, ...)
    if ( HasArtifactEquipped() ) then
        frame:m4xArtifact_PopulateValues()
    end
end

function frame:m4xEventHandler(event, ...)
    if ( HasArtifactEquipped() ) then
        self:m4xArtifact_PopulateValues()
    end
end

Colon syntax in a function implies the first argument is self. This probably won't fix your problem though as nothing in that function actually uses the event information.