Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
assertExtendedElement( module, element, propDefn );
		checkListMemberRef( ref );
		List list = ref.getList( module, element );
		if ( list == null )
			throw new PropertyValueException( element, ref.getPropDefn( ),
					null,
					PropertyValueException.DESIGN_EXCEPTION_ITEM_NOT_FOUND );
		int posn = list.indexOf( structure );
		if ( posn == -1 )
			throw new PropertyValueException( element, ref.getPropDefn( )
					.getName( ), null,
					PropertyValueException.DESIGN_EXCEPTION_ITEM_NOT_FOUND );
		doRemoveItem( new CachedMemberRef( ref, posn ) );
	}
	/**
	 * Removes structure from structure list.
	 * 
	 * @param structRef
	 *            reference to the item to remove
	 */
	private void doRemoveItem( MemberRef structRef )
	{
		String label = ModelMessages
				.getMessage( MessageConstants.REMOVE_ITEM_MESSAGE );
		ActivityStack stack = module.getActivityStack( );
		stack.startTrans( label );
		makeLocalCompositeValue( structRef );
		List list = structRef.getList( module, element );
		assert list != null;
		Structure struct = structRef.getStructure( module, element );