chore: Clean up assignment logic (#3763)

app/models/concerns/assignment_handler.rb

@@ -4,7 +4,7 @@ module AssignmentHandler
 
   included do
     before_save :ensure_assignee_is_from_team
-    after_update :notify_assignment_change, :process_assignment_activities
+    after_commit :notify_assignment_change, :process_assignment_activities
   end
 
   private

app/models/notification.rb

@@ -71,6 +71,7 @@ class Notification < ApplicationRecord
   end
 
   # TODO: move to a data presenter
+  # rubocop:disable Metrics/CyclomaticComplexity
   def push_message_title
     case notification_type
     when 'conversation_creation'
@@ -81,14 +82,15 @@ class Notification < ApplicationRecord
       I18n.t(
         'notifications.notification_title.assigned_conversation_new_message',
         display_id: conversation.display_id,
-        content: primary_actor.content&.truncate_words(10)
+        content: primary_actor&.content&.truncate_words(10)
       )
     when 'conversation_mention'
-      "[##{conversation.display_id}] #{transform_user_mention_content primary_actor.content}"
+      "[##{conversation&.display_id}] #{transform_user_mention_content primary_actor&.content}"
     else
       ''
     end
   end
+  # rubocop:enable Metrics/CyclomaticComplexity
 
   def conversation
     return primary_actor.conversation if %w[assigned_conversation_new_message conversation_mention].include? notification_type