Having ITEM_ID mapped as a column in ItemDetail is a bit odd, and mapping it that way might might be the source of the problem. Nothing is telling the ItemDetail class that that field should be populated with a proper id for the parent Item, including that it shouldn't be null.
If the detail doesn't need to know about the parent, you might be able to just omit that field in the ItemDetail java code altogether. The field in the table should be populated as a consequence of the relation.
It's more common to map this sort of thing as a bidirectional association, so that you have a @OneToMany relation of Item to ItemDetail and a @ManyToOne relation of ItemDetail to Item, and the relations can be navigated in Java. If the ItemDetail does need to know about the parent item, you should do it this way.
This is described somewhere in the Hibernate Annotations Reference section on mapping associations.