Getting NULL value when trying to trigger script assertion.

Trying to get value from XML response. But getting NULL value with script assertion passed.

Script:

import com.eviware.soapui.support.XmlHolder
import javax.xml.XMLConstants
import javax.xml.transform.stream.StreamSource
import javax.xml.validation.SchemaFactory

def holder = new XmlHolder( messageExchange.responseContentAsXml )
def val=holder.getNodeValue("//*:NDCMSG_Header//@Function")
log.info(val)

Response:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
<soap:Body>
<ns2:NDCMSG_Envelope xmlns:ns2="http://dummy.aero/NDC/NDCUtility/v2">
<NDCMSG_Header>
<Function>SOAPProject</Function>
<SchemaType>SAMPLE</SchemaType>
<SchemaVersion>2.0</SchemaVersion>

<Recipient>
<Address>
<Company>SAMPLE</Company>
<NDCSystemId>DEV</NDCSystemId>
</Address>
</Recipient>
<Sender>
<Address>
<Company>SAMPLE</Company>
<NDCSystemId>SAMPLE</NDCSystemId>
</Address>
</Sender>
</NDCMSG_Header>
<NDCMSG_Body>
<NDCMSG_Payload>
<Document>
<Name>Sample Project</Name>
<ReferenceVersion>3200</ReferenceVersion>
</Document>
<Success/>

<DataLists>
<PassengerList>
<Passenger PassengerID="WJEUSGS-T1">
<PTC>ADT</PTC>
</Passenger>
</PassengerList>
<FlightSegmentList>
<FlightSegment SegmentKey="WERSDIL-SEGMENT-1">
<Departure>
<AirportCode>MAA</AirportCode>
<Date>2019-11-29</Date>
<Time>14:30</Time>
</Departure>
<Arrival>
<AirportCode>DEL</AirportCode>
<Date>2019-11-29</Date>
<Time>17:35</Time>
</Arrival>
<MarketingCarrier>
<AirlineID>AB</AirlineID>
<Name>Southwest Airlines</Name>
<FlightNumber>100</FlightNumber>
<ResBookDesigCode>C</ResBookDesigCode>
</MarketingCarrier>
<OperatingCarrier>
<AirlineID>AB</AirlineID>
<Name>Southwest Airlines</Name>
</OperatingCarrier>
<ClassOfService>
<Code>C</Code>
</ClassOfService>
<FlightDetail>
<FlightSegmentType>30</FlightSegmentType>
<Stops>
<StopQuantity>0</StopQuantity>
</Stops>
</FlightDetail>
</FlightSegment>
</FlightSegmentList>
</DataLists>
</OfferPriceRS>
</NDCMSG_Payload>
</NDCMSG_Body>
</ns2:NDCMSG_Envelope>
</soap:Body>
</soap:Envelope>

Trying to get value of Function. Please help to get one value from the response with help of groovy script.