Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
PropertyValueException.DESIGN_EXCEPTION_NOT_LIST_TYPE );
	}
	/**
	 * Validates the values of the item members.
	 * 
	 * @param ref
	 *            reference to a list.
	 * @param item
	 *            the item to check
	 * @throws SemanticError
	 *             if element reference is invalid.
	 * @throws PropertyValueException
	 *             if the item has any member with invalid value or if the given
	 *             structure is not of a valid type that can be contained in the
	 *             list.
	 */
	private Object checkItem( ElementPropertyDefn prop, Object item )
			throws PropertyValueException, SemanticError
	{
		assert item != null;
		assert prop.getTypeCode( ) == PropertyType.LIST_TYPE;
		Object value = item;
		if ( item instanceof DesignElementHandle )
			value = ( (DesignElementHandle) item ).getElement( );
		
		PropertyType type = prop.getSubType( );
		assert type != null;
		Object result = type.validateValue( module, prop, value );
		if ( result instanceof ElementRefValue
				&& !( (ElementRefValue) result ).isResolved( ) )