Fucking finally

code/modules/client/preferences.dm

@@ -221,6 +221,7 @@ GLOBAL_LIST_EMPTY(preferences_datums)
 	var/hide_ckey = FALSE //pref for hiding if your ckey shows round-end or not
 
 	var/list/tcg_cards = list()
+	var/list/tcg_decks = list()
 
 /datum/preferences/New(client/C)
 	parent = C