raid-compact-layout: only move existing group headers

mods/raid-compact-layout.lua

@@ -40,6 +40,7 @@ module.enable = function(self)
   -- wait for the game to be loaded
   local delay = CreateFrame("Frame")
   delay:SetScript("OnUpdate", function()
+      if ShaguTweaksRaidHeaders then
         -- modify group headers
         for i=1, 8 do
           if ShaguTweaksRaidHeaders[i] then
@@ -57,6 +58,7 @@ module.enable = function(self)
             ShaguTweaksRaidHeaders[i]:SetAlpha(.75)
           end
         end
+      end
 
       -- disable delay
       this:Hide()