Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
* @param prop
	 *            definition of the simple value list property
	 * @param item
	 *            the item to be removed from the list.
	 * @throws SemanticException
	 *             if the item to remove is not found.
	 * @throws IndexOutOfBoundsException
	 *             if the given posn is out of range
	 *             <code>(index &lt; 0 || index &gt;= list.size())</code>.
	 */
	public void removeItem( ElementPropertyDefn prop, Object item )
			throws SemanticException
	{
		assert prop != null;
		if ( item == null )
			return;
		assertExtendedElement( module, element, prop );
		checkListProperty( prop );
		Object value = null;
		if ( prop.getSubTypeCode( ) == PropertyType.ELEMENT_REF_TYPE )
			value = checkItem( prop, item );
		else
			value = item;
		List list = element.getListProperty( module, prop.getName( ) );
		if ( list == null )
			throw new PropertyValueException( element, prop, null,
					PropertyValueException.DESIGN_EXCEPTION_ITEM_NOT_FOUND );
		int posn = list.indexOf( value );
		if ( posn == -1 )
			throw new PropertyValueException( element, prop.getName( ), null,