Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
DesignElement client = ref.element;
			BackRefRecord record = new StructBackRefRecord( module, struct,
					client, ref.propName );
			getActivityStack( ).execute( record );
		}
	}
	/**
	 * Replaces an item from a structure list with the new one.
	 * <ul>
	 * <li>The element must exist in the effective value for the list. This
	 * means the list must be set on this element or a ancestor element.</li>
	 * <li>If the property is set on this element, then the element is simply
	 * replaced</li>
	 * <li>If the property is set on an ancestor element, then the inherited
	 * list is first copied into this element. Then, the copy of the target item
	 * is removed from the copy of the list.</li>
	 * </ul>
	 * 
	 * @param ref
	 *            The structure list reference.
	 * @param oldItem
	 *            the old item to be replaced
	 * @param newItem
	 *            the new item reference.
	 * @throws SemanticException
	 *             if the old item is not found or this property type is not
	 *             structure list.
	 */
	public void replaceItem( MemberRef ref, IStructure oldItem,
			IStructure newItem ) throws SemanticException