Fix wall notes event trying to render target

app/roles/note_event.rb

@@ -18,4 +18,16 @@ module NoteEvent
   def note_target_id
     target.noteable_id
   end
+
+  def wall_note?
+    target.noteable_type.blank?
+  end
+
+  def note_target_type
+    if target.noteable_type.present?
+      target.noteable_type.titleize
+    else
+      "Wall"
+    end.downcase
+  end
 end

app/views/events/event/_note.html.haml

 .event-title
   %span.author_name= link_to_author event
-  %span.event_label commented on #{event.target.noteable_type_name}
-  - if event.target and event.note_target
+  %span.event_label commented on #{event.note_target_type}
+  - if event.note_target
     - if event.note_commit?
       = link_to event.note_short_commit_id, project_commit_path(event.project, event.note_commit_id), class: "commit_short_id"
     - else
       = link_to [event.project, event.note_target] do
         %strong= truncate event.note_target_id
 
+  - elsif event.wall_note?
+    -# nothing here
   - else
     %strong (deleted)
   at