fix error

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

@@ -24,7 +24,7 @@
           <% end %>
         </td>
         <td>
-          <%= b.category.title %>
+          <%= b.category.title rescue 'category not set' %>
           <div class="quick-edit">
             <ul class="nav nav-pills">
               <% if can_edit_or_delete?(b) %>