Bug 201017 TopN, TopPercent, BottomPercent filters will lead NPE when the values contains NULL[07]
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/script/NEvaluator.java
break;
			default:
				assert false;		
				return null;
		}
		
		instance.op_expr = op_expr;
		instance.n_expr = n_expr;
		instance.filterPassController = filterPassController;
		
		return instance;
	}
	
	
	/**
	 * Evaluate the given value
	 * @param value
	 * @param n
	 * @return
	 * @throws DataException
	 */
	public boolean evaluate( Context cx, Scriptable scope ) throws DataException
	{
		if( filterPassController.getForceReset() )
		{
			doReset();
			filterPassController.setForceReset( false );
		}
		
		if ( N == -1 )