Fixes manifest hard dels (#92283)

## About The Pull Request

<img width="378" height="199" alt="1eSxYbsh0e"
src="https://github.com/user-attachments/assets/e8a658ca-c1c4-48fe-bb51-c77c85a7f824"
/>

Noticed some hard dels here, does some light refactoring/code
improvement to ensure that doesn't happen.

Crates shouldn't really be owning a hard ref to the manifest in the
first place since they are detachable. Removes some code duplication in
favor of calling `tear_manifest()` which has the safety check to prevent
`forceMove()`ing a qdeleted manifest out of nullspace.

## Why It's Good For The Game

Less server hiccups.

## Changelog

Nothing players will notice besides less server hiccups.

code/game/objects/structures/crates_lockers/crates.dm

@@ -26,7 +26,7 @@
 	/// The time spent to climb this crate.
 	var/crate_climb_time = 2 SECONDS
 	/// The reference of the manifest paper attached to the cargo crate.
-	var/obj/item/paper/fluff/jobs/cargo/manifest/manifest
+	var/datum/weakref/manifest
 	/// Where the Icons for lids are located.
 	var/lid_icon = 'icons/obj/storage/crates.dmi'
 	/// Icon state to use for lid to display when opened. Leave undefined if there isn't one.
@@ -67,7 +67,7 @@
 	AddComponent(/datum/component/soapbox)
 
 /obj/structure/closet/crate/Destroy()
-	QDEL_NULL(manifest)
+	manifest = null
 	return ..()
 
 /obj/structure/closet/crate/CanAllowThrough(atom/movable/mover, border_dir)
@@ -114,8 +114,7 @@
 	. = ..()
 	if(.)
 		return
-	if(manifest)
-		tear_manifest(user)
+	tear_manifest(user)
 
 /obj/structure/closet/crate/after_open(mob/living/user, force)
 	. = ..()
@@ -126,11 +125,8 @@
 			RemoveElement(/datum/element/elevation, pixel_shift = elevation)
 		if(elevation_open)
 			AddElement(/datum/element/elevation, pixel_shift = elevation_open)
-	if(!QDELETED(manifest))
-		playsound(src, 'sound/items/poster/poster_ripped.ogg', 75, TRUE)
-		manifest.forceMove(get_turf(src))
-		manifest = null
-		update_appearance()
+
+	tear_manifest()
 
 /obj/structure/closet/crate/after_close(mob/living/user)
 	. = ..()
@@ -153,12 +149,17 @@
 
 ///Removes the supply manifest from the closet
 /obj/structure/closet/crate/proc/tear_manifest(mob/user)
-	to_chat(user, span_notice("You tear the manifest off of [src]."))
+	var/obj/item/paper/fluff/jobs/cargo/manifest/our_manifest = manifest?.resolve()
+	if(QDELETED(our_manifest))
+		manifest = null
+		return
+	if(user)
+		to_chat(user, span_notice("You tear the manifest off of [src]."))
 	playsound(src, 'sound/items/poster/poster_ripped.ogg', 75, TRUE)
 
-	manifest.forceMove(loc)
+	our_manifest.forceMove(drop_location(src))
 	if(ishuman(user))
-		user.put_in_hands(manifest)
+		user.put_in_hands(our_manifest)
 	manifest = null
 	update_appearance()