Fix runtime with NTOS apps and improve code involved (#53779)

There exists a specific state where ui.open() can be called yet the 
result would be a null window and such behaviour would be intentional. 
The following CRASH in ui.send_asset() would thus be misleading, 
because send_asset() was called after open().

This PR adds more information to the CRASH about when the failure state 
can occur, makes open() return a value based on whether it actually 
opened a new pooled window or not, and makes sure modular computer apps 
don't send_assets unless a new pooled window was created.

code/modules/modular_computers/computers/item/computer_ui.dm

@@ -37,7 +37,7 @@
 	if (!ui)
 		ui = new(user, src, "NtosMain")
 		ui.set_autoupdate(TRUE)
-		ui.open()
+		if(ui.open())
 			ui.send_asset(get_asset_datum(/datum/asset/simple/headers))
 
 

code/modules/modular_computers/file_system/program.dm

@@ -179,7 +179,7 @@
 	ui = SStgui.try_update_ui(user, src, ui)
 	if(!ui && tgui_id)
 		ui = new(user, src, tgui_id, filedesc)
-		ui.open()
+		if(ui.open())
 			ui.send_asset(get_asset_datum(/datum/asset/simple/headers))
 
 // CONVENTIONS, READ THIS WHEN CREATING NEW PROGRAM AND OVERRIDING THIS PROC:

code/modules/tgui/tgui.dm

@@ -67,18 +67,20 @@
  * public
  *
  * Open this UI (and initialize it with data).
+ *
+ * return bool - TRUE if a new pooled window is opened, FALSE in all other situations including if a new pooled window didn't open because one already exists.
  */
 /datum/tgui/proc/open()
 	if(!user.client)
-		return null
+		return FALSE
 	if(window)
-		return null
+		return FALSE
 	process_status()
 	if(status < UI_UPDATE)
-		return null
+		return FALSE
 	window = SStgui.request_pooled_window(user)
 	if(!window)
-		return null
+		return FALSE
 	opened_at = world.time
 	window.acquire_lock(src)
 	if(!window.is_ready())
@@ -101,6 +103,8 @@
 		with_static_data = TRUE))
 	SStgui.on_open(src)
 
+	return TRUE
+
 /**
  * public
  *
@@ -156,7 +160,7 @@
  */
 /datum/tgui/proc/send_asset(datum/asset/asset)
 	if(!window)
-		CRASH("send_asset() can only be called after open().")
+		CRASH("send_asset() was called either without calling open() first or when open() did not return TRUE.")
 	return window.send_asset(asset)
 
 /**