Porting "Fixes Null Reference + Qdel Issue in Runechat" (#12271)

code/datums/chatmessage.dm

@@ -35,7 +35,7 @@
   * * extra_classes - Extra classes to apply to the span that holds the text
   * * lifespan - The lifespan of the message in deciseconds
   */
-/datum/chatmessage/New(text, atom/target, mob/owner, list/extra_classes = null, lifespan = CHAT_MESSAGE_LIFESPAN)
+/datum/chatmessage/New(text, atom/target, mob/owner, list/extra_classes = list(), lifespan = CHAT_MESSAGE_LIFESPAN)
 	. = ..()
 	if (!istype(target))
 		CRASH("Invalid target given for chatmessage")
@@ -55,6 +55,12 @@
 	message = null
 	return ..()
 
+/**
+  * Calls qdel on the chatmessage when its parent is deleted, used to register qdel signal
+  */
+/datum/chatmessage/proc/on_parent_qdel()
+	qdel(src)
+
 /**
   * Generates a chat message image representation
   *
@@ -68,7 +74,7 @@
 /datum/chatmessage/proc/generate_image(text, atom/target, mob/owner, list/extra_classes, lifespan)
 	// Register client who owns this message
 	owned_by = owner.client
-	RegisterSignal(owned_by, COMSIG_PARENT_QDELETING, .proc/qdel, src)
+	RegisterSignal(owned_by, COMSIG_PARENT_QDELETING, .proc/on_parent_qdel)
 
 	// Clip message
 	var/maxlen = owned_by.prefs.max_chat_length
@@ -108,7 +114,7 @@
 	// BYOND Bug #2563917
 	// Construct text
 	var/static/regex/html_metachars = new(@"&[A-Za-z]{1,7};", "g")
-	var/complete_text = "<span class='center maptext [extra_classes != null ? extra_classes.Join(" ") : ""]' style='color: [tgt_color]'>[text]</span>"
+	var/complete_text = "<span class='center maptext [extra_classes.Join(" ")]' style='color: [tgt_color]'>[text]</span>"
 	var/mheight = WXH_TO_HEIGHT(owned_by.MeasureText(replacetext(complete_text, html_metachars, "m"), null, CHAT_MESSAGE_WIDTH))
 	approx_lines = max(1, mheight / CHAT_MESSAGE_APPROX_LHEIGHT)
 
@@ -166,7 +172,7 @@
   */
 /mob/proc/create_chat_message(atom/movable/speaker, datum/language/message_language, raw_message, list/spans, message_mode)
 	// Ensure the list we are using, if present, is a copy so we don't modify the list provided to us
-	spans = spans?.Copy()
+	spans = spans ? spans.Copy() : list()
 
 	// Check for virtual speakers (aka hearing a message through a radio)
 	var/atom/movable/originalSpeaker = speaker