Merge pull request #11807 from Citadel-Station-13/kevinz000-patch-4

datum pipeline null gasmix stack trace now gives a REF()

code/modules/atmospherics/machinery/datum_pipeline.dm

@@ -207,7 +207,7 @@
 /datum/pipeline/proc/return_air()
 	. = other_airs + air
 	if(null in .)
-		stack_trace("[src] has one or more null gas mixtures, which may cause bugs. Null mixtures will not be considered in reconcile_air().")
+		stack_trace("[src]([REF(src)]) has one or more null gas mixtures, which may cause bugs. Null mixtures will not be considered in reconcile_air().")
 		return removeNullsFromList(.)
 
 /datum/pipeline/proc/reconcile_air()