NullReferenceException unhandled by user code while changing XML document

I am getting the error : NullReferenceException was unhandled by user code , object reference not set to an instance of an object. While I want to change the element of an xml file. this is the code the error is generated on :

doc2.SelectSingleNode("/Document/CstmrCdtTrfInitn/GrpHdr/MsgId", nsmgr).InnerText = bank.Afkorting + "-0001";

This is the previous code :

XmlDocument doc2 = new XmlDocument();
        doc2.Load(@"C:\Users\Kevin\Desktop\xsd\betalingen_sepa.xml");

        XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc2.NameTable);
        nsmgr.AddNamespace("xsi", "http://ift.tt/ra1lAU");

        DagboekFinancieel bank = os.FindObject<DagboekFinancieel>(CriteriaOperator.Parse("[Omschrijving] = ?", opdracht.Bank));
        doc2.SelectSingleNode("/Document/CstmrCdtTrfInitn/GrpHdr/MsgId", nsmgr).InnerText = bank.Afkorting + "-0001";

        string naamFile = bank.Afkorting + "-0001";
        doc2.Save(@"C:\" + naamFile + ".xml");

This is the original XML file i got :

<?xml version="1.0" encoding="UTF-8"?>

<Document xmlns:xsi="http://ift.tt/ra1lAU" xmlns="urn:iso:std:iso:20022:tech:xsd:pain.001.001.03">


<CstmrCdtTrfInitn>
<GrpHdr>
<MsgId>@ref_xml@</MsgId>
<CreDtTm>@datum_tijd_xml@</CreDtTm>
<NbOfTxs>@aantal_transacties@</NbOfTxs>
<InitgPty>
<Nm>@afzender_naam@</Nm>
<Id>
<OrgId>
<Othr>
<Id>@afzender_btw_nr@</Id>
<Issr>KBO-BCE</Issr>
</Othr>
</OrgId>
</Id>
</InitgPty>
</GrpHdr>
<PmtInf>
<PmtInfId>@ref_payment_block@</PmtInfId>
<PmtMtd>TRF</PmtMtd>
<BtchBookg>true</BtchBookg>
<PmtTpInf>
<SvcLvl>
<Cd>SEPA</Cd>
</SvcLvl>
</PmtTpInf>
<ReqdExctnDt>@datum_uitvoering@</ReqdExctnDt>
<Dbtr>
<Nm>@afzender_naam@</Nm>
</Dbtr>
<DbtrAcct>
<Id>
<IBAN>@afzender_iban@</IBAN>
</Id>
</DbtrAcct>
<DbtrAgt>
<FinInstnId>
<BIC>@afzender_bic@</BIC>
</FinInstnId>
</DbtrAgt>
<ChrgBr>@kosten@</ChrgBr>
<CdtTrfTxInf>
<PmtId>
<EndToEndId>@ref_end_to_end@</EndToEndId>
</PmtId>
<Amt>
<InstdAmt Ccy="@munt@">@bedrag@</InstdAmt>
</Amt>
<CdtrAgt>
<FinInstnId>
<BIC>@leveran_bic@</BIC>
</FinInstnId>
</CdtrAgt>
<Cdtr>
<Nm>@leveran_naam@</Nm>
<PstlAdr>
<Ctry>@leveran_land@</Ctry>
<AdrLine>@leveran_straat@</AdrLine>
<AdrLine>@leveran_wpl@</AdrLine>
</PstlAdr>
</Cdtr>
<CdtrAcct>
<Id>
<IBAN>@leveran_iban@</IBAN>
</Id>
</CdtrAcct>
<RmtInf>
<Ustrd>@leveran_ref@</Ustrd>
<Strd>
<CdtrRefInf>
<Tp>
<CdOrPrtry>
<Cd>@gestruct_mededeling_soort1@</Cd>
</CdOrPrtry>
<Issr>@gestruct_mededeling_soort2@</Issr>
</Tp>
<Ref>@gestruct_mededeling@</Ref>
</CdtrRefInf>
</Strd>
</RmtInf>
</CdtTrfTxInf>
</PmtInf>
</CstmrCdtTrfInitn>
</Document>