fixed inline thread replace error

lms/static/coffee/src/discussion/views/discussion_thread_show_view.coffee

@@ -125,6 +125,7 @@ class @DiscussionThreadShowView extends DiscussionContentView
         @model.set('endorsed', not endorsed)
 
   highlight: (el) ->
+    if el.html()
       el.html(el.html().replace(/&lt;mark&gt;/g, "<mark>").replace(/&lt;\/mark&gt;/g, "</mark>"))
 
 class @DiscussionThreadInlineShowView extends DiscussionThreadShowView