FIX: Topic and Post may be `nil`.

app/jobs/regular/user_email.rb

@@ -50,7 +50,7 @@ module Jobs
 
     NOTIFICATIONS_SENT_BY_MAILING_LIST ||= Set.new %w{posted replied mentioned group_mentioned quoted}
 
-   def message_for_email(user, post, type, notification,
+    def message_for_email(user, post, type, notification,
                          notification_type=nil, notification_data_hash=nil,
                          email_token=nil, to_address=nil)
 
@@ -86,7 +86,7 @@ module Jobs
         end
 
         if user.user_option.mailing_list_mode? &&
-           !post.topic.private_message? &&
+           (!post.try(:topic).try(:private_message?)) &&
            NOTIFICATIONS_SENT_BY_MAILING_LIST.include?(email_args[:notification_type])
            # no need to log a reason when the mail was already sent via the mailing list job
            return [nil, nil]

spec/jobs/user_email_spec.rb

@@ -200,6 +200,12 @@ describe Jobs::UserEmail do
         Jobs::UserEmail.new.execute(type: :user_mentioned, user_id: user.id, notification_id: notification.id, post_id: post.id)
         # other times, we only pass the type of notification
         Jobs::UserEmail.new.execute(type: :user_mentioned, user_id: user.id, notification_type: "posted", post_id: post.id)
+        # When post is nil
+        Jobs::UserEmail.new.execute(type: :user_mentioned, user_id: user.id, notification_type: "posted")
+        # When post does not have a topic
+        post = Fabricate(:post)
+        post.topic.destroy
+        Jobs::UserEmail.new.execute(type: :user_mentioned, user_id: user.id, notification_type: "posted", post_id: post)
       end
 
       it "doesn't send the email if the post has been user deleted" do