Disable sequence nav buttons when the sequence is empty

common/lib/xmodule/xmodule/js/src/sequence/display.coffee

@@ -56,7 +56,7 @@ class @Sequence
       element.removeClass('progress-none')
              .removeClass('progress-some')
              .removeClass('progress-done')
-      
+
       switch progress
         when 'none' then element.addClass('progress-none')
         when 'in_progress' then element.addClass('progress-some')
@@ -65,6 +65,11 @@ class @Sequence
   toggleArrows: =>
     @$('.sequence-nav-buttons a').unbind('click')
 
+    if @contents.length == 0
+      @$('.sequence-nav-buttons .prev a').addClass('disabled')
+      @$('.sequence-nav-buttons .next a').addClass('disabled')
+      return
+
     if @position == 1
       @$('.sequence-nav-buttons .prev a').addClass('disabled')
     else
@@ -105,8 +110,8 @@ class @Sequence
 
     if (1 <= new_position) and (new_position <= @num_contents)
       Logger.log "seq_goto", old: @position, new: new_position, id: @id
-    
-      # On Sequence chage, destroy any existing polling thread 
+
+      # On Sequence chage, destroy any existing polling thread
       #   for queued submissions, see ../capa/display.coffee
       if window.queuePollerID
         window.clearTimeout(window.queuePollerID)