FIX: Use a proper event for removing assign on close

lib/topic_assigner.rb

@@ -44,12 +44,6 @@ SQL
   end
 
   def self.auto_assign(post, force: false)
-
-    if SiteSetting.unassign_on_close && post.topic && post.topic.closed
-      assigner = new(post.topic, Discourse.system_user)
-      assigner.unassign(silent: true)
-    end
-
     return unless SiteSetting.assigns_by_staff_mention
 
     if post.user && post.topic && post.user.staff?

plugin.rb

@@ -174,6 +174,13 @@ after_initialize do
     ::TopicAssigner.auto_assign(post, force: true)
   end
 
+  on(:topic_closed) do |topic|
+    if SiteSetting.unassign_on_close
+      assigner = ::TopicAssigner.new(topic, Discourse.system_user)
+      assigner.unassign(silent: true)
+    end
+  end
+
   on(:move_to_inbox) do |info|
     if SiteSetting.unassign_on_group_archive && info[:group]
       if topic = info[:topic]