Fixes storage UI breaking upon an observer detaching while having your storage UI open under odd circumstances (#86842)

## About The Pull Request
This is one of the stupidest issues I had the displeasure of working
with and I still do not know concrete details behind the issue because
some of the code introduced in this (supposed-to-be) debug PR
accidentally fixed said issue. Essentially, under odd circumstances of
the user either being SSD or having their storage focus stolen by
something else, an observer who had their storage UI open via
autoobserve could break their UI by moving away because that improperly
removed the user from some lists. This is the primary theory judging
from what fixed the issue, but sadly there is no concrete info behind it
as all attempts to reproduce the bug, both locally and on live, have
failed.

Closes #85259

## Changelog
🆑
fix: Storage UI should no longer (not so much) randomly disappear,
hooray!
/🆑

code/datums/storage/storage.dm

@@ -974,7 +974,7 @@ GLOBAL_LIST_EMPTY(cached_storage_typecaches)
 		if(user.active_storage == src && user.client)
 			seeing += user
 		else
-			is_using -= user
+			hide_contents(user)
 	return seeing
 
 /**
@@ -1032,8 +1032,6 @@ GLOBAL_LIST_EMPTY(cached_storage_typecaches)
  * * mob/to_hide - the mob to hide the storage from
  */
 /datum/storage/proc/hide_contents(mob/to_hide)
-	if(!to_hide.client)
-		return TRUE
 	if(to_hide.active_storage == src)
 		to_hide.active_storage = null
 
@@ -1046,8 +1044,9 @@ GLOBAL_LIST_EMPTY(cached_storage_typecaches)
 
 	is_using -= to_hide
 
-	to_hide.client.screen -= storage_interfaces[to_hide].list_ui_elements()
-	to_hide.client.screen -= real_location.contents
+	if(to_hide.client)
+		to_hide.client.screen -= storage_interfaces[to_hide].list_ui_elements()
+		to_hide.client.screen -= real_location.contents
 	QDEL_NULL(storage_interfaces[to_hide])
 	storage_interfaces -= to_hide
 
@@ -1078,7 +1077,9 @@ GLOBAL_LIST_EMPTY(cached_storage_typecaches)
 	var/columns = clamp(max_slots, 1, screen_max_columns)
 	var/rows = clamp(CEILING(adjusted_contents / columns, 1) + additional_row, 1, screen_max_rows)
 
-	for (var/ui_user in storage_interfaces)
+	for (var/mob/ui_user as anything in storage_interfaces)
+		if (isnull(storage_interfaces[ui_user]))
+			continue
 		storage_interfaces[ui_user].update_position(screen_start_x, screen_pixel_x, screen_start_y, screen_pixel_y, columns, rows)
 
 	var/current_x = screen_start_x