fix error caused by no title saved.

app/views/admin/announcements/_index.html.erb

@@ -24,9 +24,9 @@
         </td>
         <td>
           <% if b.expired? || (b.category.disable rescue false)%>
-            <%= b.title.html_safe %>
+            <%= b.title.to_s.html_safe %>
           <% else %>
-            <a href="<%= page_for_bulletin(b) %>" target="_blank"><%= b.title.html_safe %></a> 
+            <a href="<%= page_for_bulletin(b) %>" target="_blank"><%= b.title.to_s.html_safe %></a> 
           <% end %>
 
           <% if b.expired? %>