FIX: Handle deleted original message for thread index (#21470)

Since we have channel message retention which deletes
messages, we can end up with cases where the thread
is still around but the message is deleted. We will
handle the cascade delete in a different commit --
for now we will ensure the thread list lookup handles
this case and doesn't error.

plugins/chat/app/serializers/chat/thread_serializer.rb

@@ -10,7 +10,9 @@ module Chat
     def initialize(object, opts)
       super(object, opts)
       @opts = opts
-      original_message.thread = object
+
+      # Avoids an N1 to re-load the thread in the serializer for original_message.
+      object.original_message.thread = object
     end
 
     def meta

plugins/chat/app/services/chat/lookup_channel_threads.rb

@@ -64,7 +64,9 @@ module Chat
         )
         .where("chat_messages.user_id = ? OR chat_messages.user_id IS NULL", guardian.user.id)
         .where(channel_id: channel.id)
-        .where("original_messages.deleted_at IS NULL AND chat_messages.deleted_at IS NULL")
+        .where(
+          "original_messages.deleted_at IS NULL AND chat_messages.deleted_at IS NULL AND original_messages.id IS NOT NULL",
+        )
         .group("chat_threads.id")
         .order("last_posted_at DESC NULLS LAST")
         .limit(50)