Fix more Destroy / GC issues.

Fixes recursive Destroy call on containment field.
Fix unable to GC issue on air alarms.
Fix unable to GC issue on radiation sources.

code/datums/repositories/radiation.dm

@@ -18,7 +18,7 @@ var/global/repository/radiation/radiation_repository = new()
 /datum/radiation_source/Destroy()
 	radiation_repository.sources -= src
 	if(radiation_repository.sources_assoc[src.source_turf] == src)
-		radiation_repository.sources -= src.source_turf
+		radiation_repository.sources_assoc -= src.source_turf
 	src.source_turf = null
 	. = ..()
 

code/game/machinery/alarm.dm

@@ -103,6 +103,9 @@
 	unregister_radio(src, frequency)
 	qdel(wires)
 	wires = null
+	if(alarm_area && alarm_area.master_air_alarm == src)
+		alarm_area.master_air_alarm = null
+		elect_master(exclude_self = TRUE)
 	return ..()
 
 /obj/machinery/alarm/New()
@@ -271,8 +274,10 @@
 /obj/machinery/alarm/proc/master_is_operating()
 	return alarm_area && alarm_area.master_air_alarm && !(alarm_area.master_air_alarm.stat & (NOPOWER | BROKEN))
 
-/obj/machinery/alarm/proc/elect_master()
+/obj/machinery/alarm/proc/elect_master(exclude_self = FALSE)
 	for(var/obj/machinery/alarm/AA in alarm_area)
+		if(exclude_self && AA == src)
+			continue
 		if(!(AA.stat & (NOPOWER|BROKEN)))
 			alarm_area.master_air_alarm = AA
 			return 1

code/modules/power/singularity/containment_field.dm

@@ -20,7 +20,7 @@
 		FG1.cleanup()
 	if(FG2 && !FG2.clean_up)
 		FG2.cleanup()
-	..()
+	. = ..()
 
 /obj/machinery/containment_field/attack_hand(mob/user as mob)
 	if(get_dist(src, user) > 1)

code/modules/power/singularity/field_generator.dm

@@ -168,7 +168,7 @@ field_generator power level display
 
 /obj/machinery/field_generator/Destroy()
 	src.cleanup()
-	..()
+	. = ..()
 
 
 
@@ -312,12 +312,12 @@ field_generator power level display
 /obj/machinery/field_generator/proc/cleanup()
 	clean_up = 1
 	for (var/obj/machinery/containment_field/F in fields)
-		if (isnull(F))
+		if (QDELETED(F))
 			continue
 		qdel(F)
 	fields = list()
 	for(var/obj/machinery/field_generator/FG in connected_gens)
-		if (isnull(FG))
+		if (QDELETED(FG))
 			continue
 		FG.connected_gens.Remove(src)
 		if(!FG.clean_up)//Makes the other gens clean up as well