stops ducts from constantly runtiming on being placed down

credits to steamport for finding it, I completely and utterly forgot a pretty obvious mistake

code/modules/plumbing/ducts.dm

@@ -66,7 +66,7 @@ All the important duct code:
 
 	for(var/atom/movable/AM in loc)
 		var/datum/component/plumbing/P = AM.GetComponent(/datum/component/plumbing)
-		if(P.active)
+		if(P?.active)
 			disconnect_duct() //let's not built under plumbing machinery
 			return
 	for(var/D in GLOB.cardinals)