Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
*             structure is not of a valid type that can be contained in the
	 *             list.
	 */
	private void checkItem( MemberRef ref, IStructure item )
			throws SemanticException
	{
		assert item != null;
		PropertyDefn propDefn = null;
		if ( ref.refType == MemberRef.PROPERTY )
		{
			propDefn = ref.getPropDefn( );
			if ( item.getDefn( ) != propDefn.getStructDefn( ) )
			{
				throw new PropertyValueException( element, propDefn, item,
						PropertyValueException.DESIGN_EXCEPTION_WRONG_ITEM_TYPE );
			}
		}
		else
		{
			propDefn = ref.getMemberDefn( );
			if ( item.getDefn( ) != propDefn.getStructDefn( ) )
			{
				throw new PropertyValueException( element, ref.getPropDefn( ),
						propDefn, item,
						PropertyValueException.DESIGN_EXCEPTION_WRONG_ITEM_TYPE );
			}
		}
		for ( Iterator iter = item.getDefn( ).propertiesIterator( ); iter
				.hasNext( ); )
		{