asset cache + better minimaps

code/controllers/subsystem/minimaps.dm

@@ -1,8 +1,8 @@
 SUBSYSTEM_DEF(minimaps)
 	name = "Minimaps"
 	flags = SS_NO_FIRE
-	var/list/station_minimaps
-	var/datum/minimap_group/station_minimap
+	var/list/station_minimaps = list()
+	var/datum/minimap_group/station_minimap = null
 
 /datum/controller/subsystem/minimaps/Initialize()
 	if(!CONFIG_GET(flag/minimaps_enabled))
@@ -12,9 +12,9 @@ SUBSYSTEM_DEF(minimaps)
 	return ..()
 
 /datum/controller/subsystem/minimaps/proc/build_minimaps()
-	station_minimaps = list()
 	for(var/z in SSmapping.levels_by_trait(ZTRAIT_STATION))
 		var/datum/space_level/SL = SSmapping.get_level(z)
 		var/name = (SL.name == initial(SL.name))? "[z] - Station" : "[z] - [SL.name]"
 		station_minimaps += new /datum/minimap(z, name = name)
+
 	station_minimap = new(station_minimaps, "Station")