act有可能为空，导致500错误

app/views/courses/_course_activity.html.erb

@@ -94,7 +94,7 @@
                 $('#reply_content_<%= activity.id %> a').colorbox({rel:'nofollow', close: "关闭", returnFocus: false});
             });
         </script>
-            <% if activity %>
+            <% if activity && activity.course_act%>
                 <% act = activity.course_act %>
                 <% case activity.course_act_type.to_s %>
                 <% when 'HomeworkCommon' %>