Added check for existence of addon.Frame to addon:TRADE_SKILL_CLOSE()

diff --git a/core.lua b/core.lua
index cb872d2..01ee658 100644
--- a/core.lua
+++ b/core.lua
@@ -789,7 +789,7 @@ do
 end

 function addon:TRADE_SKILL_CLOSE()
-	if addon.db.profile.closeguionskillclose then
+	if self.Frame and addon.db.profile.closeguionskillclose then
 		self.Frame:Hide()
 	end