GetName Error

[Yafis]

Hi! How to do a background check so as not to give an error when using :GetName() in the Game Shop or Send Twitter
My Lua code:

local SFN = CreateFrame("Frame")
SFN:SetScript("OnUpdate", function()
    local f = GetMouseFocus()
    if f then
        local n = f:GetName() or tostring(f)
        if n then
            FramerateLabel:SetText(n.." - FPS: ")
        end
    end
end)

Error:

Message: Interface\AddOns\WorldTools\Libs\lib_hook.lua:29: Attempt to access forbidden object from code tainted by an AddOn
Time: 04/22/15 04:48:51
Count: 182
Stack: [C]: in function `GetName'
Interface\AddOns\WorldTools\Libs\lib_hook.lua:29: in function <Interface\AddOns\WorldTools\Libs\lib_hook.lua:26>

Locals: (*temporary) = <unnamed> {
 SplashBanner = <unnamed> {
 }
 CurrentMarketPrice = <unnamed> {
 }
 NormalPrice = <unnamed> {
 }
 InvisibleMouseOverFrame = <unnamed> {
 }
 Strikethrough = <unnamed> {
 }
 GlowPulse = <unnamed> {
 }
 IconBorder = <unnamed> {
 }
 SplashBannerText = <unnamed> {
 }
 Icon = <unnamed> {
 }
 UpgradeArrow = <unnamed> {
 }
 Shadows = <unnamed> {
 }
 BuyButton = <unnamed> {
 }
 ProductName = <unnamed> {
 }
 Checkmark = <unnamed> {
 }
 Magnifier = <unnamed> {
 }
 Model = <unnamed> {
 }
 BannerFadeIn = <unnamed> {
 }
 0 = <userdata>
 Card = <unnamed> {
 }
 GlowSpin = <unnamed> {
 }
 Description = <unnamed> {
 }
 CurrentPrice = <unnamed> {
 }
 SalePrice = <unnamed> {
 }
}