HTTP Status 500 - tag 'select', field 'list', name 'post.department.depId': The requested list key 'AllDepartment' could not be resolved as a collection/array/map/enumeration/iterator type. Example: people or people.{name} - [unknown location]

原因：

由於我的action執行完又回到當前頁面.而當前頁面的<s:select list="myList">當頁面載入的時候就顯示,而我action執行完並沒有儲存myList,所以這個時候的list是空的,所以could not be resolved as a collection/array/map/enumeration/iterator type.