Null reference exception when encoding SET OF INTEGER

[DerekJergensen]

Trying to encode this ASN.1 sequence

AuthorizationList ::= SEQUENCE {
    purpose [1] SET OF INTEGER OPTIONAL,
    algorithm [2] INTEGER OPTIONAL,
    keySize [3] INTEGER OPTIONAL,
    digest [5] SET OF INTEGER OPTIONAL,
    ecCurve [10] INTEGER OPTIONAL,
    noAuthRequired [503] NULL OPTIONAL,
    creationDateTime [701] INTEGER OPTIONAL,
    origin [702] INTEGER OPTIONAL,
    rootOfTrust [704] RootOfTrust OPTIONAL,
    osVersion [705] INTEGER OPTIONAL,
    osPatchLevel [706] INTEGER OPTIONAL,
    attestationApplicationId [709] OCTET STRING OPTIONAL,
    attestationIdSerial [713] OCTET STRING OPTIONAL,
    vendorPatchLevel [718] INTEGER OPTIONAL,
    bootPatchLevel [719] INTEGER OPTIONAL
}

I used the compiler to turn that into this class

//
// This file was generated by the BinaryNotes compiler (created by Abdulla Abdurakhmanov, modified by Sylvain Prevost).
// See http://bnotes.sourceforge.net 
// Any modifications to this file will be lost upon recompilation of the source ASN.1. 
//

using System;
using System.Numerics;

using org.bn.attributes;
using org.bn.attributes.constraints;
using org.bn.coders;
using org.bn.types;
using org.bn;

namespace Test
{


    [ASN1PreparedElement]
    [ASN1Sequence(Name = "AuthorizationList", IsSet = false)]
    public class AuthorizationList : IASN1PreparedElement
    {

        private ICollection<BigInteger> purpose_;

        private bool purpose_present = false;
        [ASN1Integer(Name = "")]

        [ASN1SequenceOf(Name = "purpose", IsSetOf = true)]

        [ASN1Element(Name = "purpose", IsOptional = true, HasTag = true, Tag = 1, TagClass = TagClasses.ContextSpecific, IsImplicitTag = false, HasDefaultValue = false)]
        public ICollection<BigInteger> Purpose
        {
            get { return purpose_; }
            set { purpose_ = value; purpose_present = true; }
        }

        private BigInteger algorithm_;

        private bool algorithm_present = false;
        [ASN1Integer(Name = "")]

        [ASN1Element(Name = "algorithm", IsOptional = true, HasTag = true, Tag = 2, TagClass = TagClasses.ContextSpecific, IsImplicitTag = false, HasDefaultValue = false)]
        public BigInteger Algorithm
        {
            get { return algorithm_; }
            set { algorithm_ = value; algorithm_present = true; }
        }

        private BigInteger keySize_;

        private bool keySize_present = false;
        [ASN1Integer(Name = "")]

        [ASN1Element(Name = "keySize", IsOptional = true, HasTag = true, Tag = 3, TagClass = TagClasses.ContextSpecific, IsImplicitTag = false, HasDefaultValue = false)]
        public BigInteger KeySize
        {
            get { return keySize_; }
            set { keySize_ = value; keySize_present = true; }
        }

        private ICollection<BigInteger> digest_;

        private bool digest_present = false;
        [ASN1Integer(Name = "")]

        [ASN1SequenceOf(Name = "digest", IsSetOf = true)]

        [ASN1Element(Name = "digest", IsOptional = true, HasTag = true, Tag = 5, TagClass = TagClasses.ContextSpecific, IsImplicitTag = false, HasDefaultValue = false)]
        public ICollection<BigInteger> Digest
        {
            get { return digest_; }
            set { digest_ = value; digest_present = true; }
        }

        private BigInteger ecCurve_;

        private bool ecCurve_present = false;
        [ASN1Integer(Name = "")]

        [ASN1Element(Name = "ecCurve", IsOptional = true, HasTag = true, Tag = 10, TagClass = TagClasses.ContextSpecific, IsImplicitTag = false, HasDefaultValue = false)]
        public BigInteger EcCurve
        {
            get { return ecCurve_; }
            set { ecCurve_ = value; ecCurve_present = true; }
        }

        private NullObject noAuthRequired_;

        private bool noAuthRequired_present = false;

        [ASN1Null(Name = "noAuthRequired")]

        [ASN1Element(Name = "noAuthRequired", IsOptional = true, HasTag = true, Tag = 503, TagClass = TagClasses.ContextSpecific, IsImplicitTag = false, HasDefaultValue = false)]
        public NullObject NoAuthRequired
        {
            get { return noAuthRequired_; }
            set { noAuthRequired_ = value; noAuthRequired_present = true; }
        }

        private BigInteger creationDateTime_;

        private bool creationDateTime_present = false;
        [ASN1Integer(Name = "")]

        [ASN1Element(Name = "creationDateTime", IsOptional = true, HasTag = true, Tag = 701, TagClass = TagClasses.ContextSpecific, IsImplicitTag = false, HasDefaultValue = false)]
        public BigInteger CreationDateTime
        {
            get { return creationDateTime_; }
            set { creationDateTime_ = value; creationDateTime_present = true; }
        }

        private BigInteger origin_;

        private bool origin_present = false;
        [ASN1Integer(Name = "")]

        [ASN1Element(Name = "origin", IsOptional = true, HasTag = true, Tag = 702, TagClass = TagClasses.ContextSpecific, IsImplicitTag = false, HasDefaultValue = false)]
        public BigInteger Origin
        {
            get { return origin_; }
            set { origin_ = value; origin_present = true; }
        }

        private RootOfTrust rootOfTrust_;

        private bool rootOfTrust_present = false;

        [ASN1Element(Name = "rootOfTrust", IsOptional = true, HasTag = true, Tag = 704, TagClass = TagClasses.ContextSpecific, IsImplicitTag = false, HasDefaultValue = false)]
        public RootOfTrust RootOfTrust
        {
            get { return rootOfTrust_; }
            set { rootOfTrust_ = value; rootOfTrust_present = true; }
        }

        private BigInteger osVersion_;

        private bool osVersion_present = false;
        [ASN1Integer(Name = "")]

        [ASN1Element(Name = "osVersion", IsOptional = true, HasTag = true, Tag = 705, TagClass = TagClasses.ContextSpecific, IsImplicitTag = false, HasDefaultValue = false)]
        public BigInteger OsVersion
        {
            get { return osVersion_; }
            set { osVersion_ = value; osVersion_present = true; }
        }

        private BigInteger osPatchLevel_;

        private bool osPatchLevel_present = false;
        [ASN1Integer(Name = "")]

        [ASN1Element(Name = "osPatchLevel", IsOptional = true, HasTag = true, Tag = 706, TagClass = TagClasses.ContextSpecific, IsImplicitTag = false, HasDefaultValue = false)]
        public BigInteger OsPatchLevel
        {
            get { return osPatchLevel_; }
            set { osPatchLevel_ = value; osPatchLevel_present = true; }
        }

        private byte[] attestationApplicationId_;

        private bool attestationApplicationId_present = false;
        [ASN1OctetString(Name = "")]

        [ASN1Element(Name = "attestationApplicationId", IsOptional = true, HasTag = true, TagClass = TagClasses.ContextSpecific, Tag = 709, IsImplicitTag = false, HasDefaultValue = false)]
        public byte[] AttestationApplicationId
        {
            get { return attestationApplicationId_; }
            set { attestationApplicationId_ = value; attestationApplicationId_present = true; }
        }

        private byte[] attestationIdSerial_;

        private bool attestationIdSerial_present = false;
        [ASN1OctetString(Name = "")]

        [ASN1Element(Name = "attestationIdSerial", IsOptional = true, HasTag = true, Tag = 713, TagClass = TagClasses.ContextSpecific, IsImplicitTag = false, HasDefaultValue = false)]
        public byte[] AttestationIdSerial
        {
            get { return attestationIdSerial_; }
            set { attestationIdSerial_ = value; attestationIdSerial_present = true; }
        }

        private BigInteger vendorPatchLevel_;

        private bool vendorPatchLevel_present = false;
        [ASN1Integer(Name = "")]

        [ASN1Element(Name = "vendorPatchLevel", IsOptional = true, HasTag = true, Tag = 718, TagClass = TagClasses.ContextSpecific, IsImplicitTag = false, HasDefaultValue = false)]
        public BigInteger VendorPatchLevel
        {
            get { return vendorPatchLevel_; }
            set { vendorPatchLevel_ = value; vendorPatchLevel_present = true; }
        }

        private BigInteger bootPatchLevel_;

        private bool bootPatchLevel_present = false;
        [ASN1Integer(Name = "")]

        [ASN1Element(Name = "bootPatchLevel", IsOptional = true, HasTag = true, Tag = 719, TagClass = TagClasses.ContextSpecific, IsImplicitTag = false, HasDefaultValue = false)]
        public BigInteger BootPatchLevel
        {
            get { return bootPatchLevel_; }
            set { bootPatchLevel_ = value; bootPatchLevel_present = true; }
        }

        public bool isPurposePresent()
        {
            return purpose_present == true;
        }

        public bool isAlgorithmPresent()
        {
            return algorithm_present == true;
        }

        public bool isKeySizePresent()
        {
            return keySize_present == true;
        }

        public bool isDigestPresent()
        {
            return digest_present == true;
        }

        public bool isEcCurvePresent()
        {
            return ecCurve_present == true;
        }

        public bool isCreationDateTimePresent()
        {
            return creationDateTime_present == true;
        }

        public bool isOriginPresent()
        {
            return origin_present == true;
        }

        public bool isRootOfTrustPresent()
        {
            return rootOfTrust_present == true;
        }

        public bool isOsVersionPresent()
        {
            return osVersion_present == true;
        }

        public bool isOsPatchLevelPresent()
        {
            return osPatchLevel_present == true;
        }

        public bool isAttestationApplicationIdPresent()
        {
            return attestationApplicationId_present == true;
        }

        public bool isAttestationIdSerialPresent()
        {
            return attestationIdSerial_present == true;
        }

        public bool isVendorPatchLevelPresent()
        {
            return vendorPatchLevel_present == true;
        }

        public bool isBootPatchLevelPresent()
        {
            return bootPatchLevel_present == true;
        }


        public void initWithDefaults()
        {

        }

        private static IASN1PreparedElementData preparedData = CoderFactory.getInstance().newPreparedElementData(typeof(AuthorizationList));
        public IASN1PreparedElementData PreparedData
        {
            get { return preparedData; }
        }

    }

}

When I was trying to figure it out myself I debugged the project and saw that the null reference exception was happening on this line when trying to encode the Digest field.

I'm sure I'm just missing something small, but I would really appreciate some help or advice. Thanks!