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  <xs:element id="next_TcfCreditFacilityMember" abstract="true" name="TcfCreditFacilityMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A6.67SeniorSecuredNotesMember" abstract="true" name="A6.67SeniorSecuredNotesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A6.85SeniorSecuredNotesMember" abstract="true" name="A6.85SeniorSecuredNotesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A6.58SeniorSecuredNotesMember" abstract="true" name="A6.58SeniorSecuredNotesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A6.72SeniorSecuredLoansMember" abstract="true" name="A6.72SeniorSecuredLoansMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A7.11SeniorSecuredLoansMember" abstract="true" name="A7.11SeniorSecuredLoansMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A12.00CorporateCreditAgreementMember" abstract="true" name="A12.00CorporateCreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_A13.00SuperFinCoTermLoanMember" abstract="true" name="A13.00SuperFinCoTermLoanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_CorporateCreditAgreementMember" abstract="true" name="CorporateCreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_CorporateCreditAmendmentMember" abstract="true" name="CorporateCreditAmendmentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_SuperFinCoTermLoanMember" abstract="true" name="SuperFinCoTermLoanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_FinCoCreditAgreementMember" abstract="true" name="FinCoCreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_Train4LLCCreditAgreementMember" abstract="true" name="Train4LLCCreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_CdCreditFacilityAndTcfCreditFacilityMember" abstract="true" name="CdCreditFacilityAndTcfCreditFacilityMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_CdSeniorWorkingCapitalFacilityMember" abstract="true" name="CdSeniorWorkingCapitalFacilityMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_InitialWarrantsMember" abstract="true" name="InitialWarrantsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_CCAmendmentWarrantsMember" abstract="true" name="CCAmendmentWarrantsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_VariableRateComponentAxis" abstract="true" name="VariableRateComponentAxis" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:dimensionItem" type="xbrli:stringItemType"/>
  <xs:element id="next_VariableRateComponentDomain" abstract="true" name="VariableRateComponentDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_VariableRateComponentOneMember" abstract="true" name="VariableRateComponentOneMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_VariableRateComponentTwoMember" abstract="true" name="VariableRateComponentTwoMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_VariableRateComponentThreeMember" abstract="true" name="VariableRateComponentThreeMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_DebtInstrumentCovenantPercentageHedged" abstract="false" name="DebtInstrumentCovenantPercentageHedged" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="next_DebtInstrumentCovenantInterestCoverageRatioMinimum" abstract="false" name="DebtInstrumentCovenantInterestCoverageRatioMinimum" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:pureItemType"/>
  <xs:element id="next_DebtInstrumentCovenantRepurchasePricePercentage" abstract="false" name="DebtInstrumentCovenantRepurchasePricePercentage" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="next_PaidInKindInterestElectedNumberOfAdditionalInterestPayments" abstract="false" name="PaidInKindInterestElectedNumberOfAdditionalInterestPayments" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="next_PercentageOfPaymentsEligibleToBePaidInKind" abstract="false" name="PercentageOfPaymentsEligibleToBePaidInKind" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="next_WarrantNumberOfTranches" abstract="false" name="WarrantNumberOfTranches" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="next_LineOfCreditPeriodAfterWhichAmortizationBegins" abstract="false" name="LineOfCreditPeriodAfterWhichAmortizationBegins" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="next_LineOfCreditOptionToExtendPeriod" abstract="false" name="LineOfCreditOptionToExtendPeriod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="next_LineOfCreditExtensionOptionAvailablePeriod" abstract="false" name="LineOfCreditExtensionOptionAvailablePeriod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="next_DebtInstrumentCovenantQuarterlyExcessCashFlowSweepPercentage" abstract="false" name="DebtInstrumentCovenantQuarterlyExcessCashFlowSweepPercentage" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="next_DebtInstrumentCovenantCumulativePrepaymentsOrCancelledCommitmentsLimitPercentage" abstract="false" name="DebtInstrumentCovenantCumulativePrepaymentsOrCancelledCommitmentsLimitPercentage" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="next_WarrantTrancheAxis" abstract="true" name="WarrantTrancheAxis" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:dimensionItem" type="xbrli:stringItemType"/>
  <xs:element id="next_WarrantTrancheDomain" abstract="true" name="WarrantTrancheDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_WarrantTrancheAMember" abstract="true" name="WarrantTrancheAMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_WarrantTrancheBMember" abstract="true" name="WarrantTrancheBMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_NumberOfDaysUsedToDetermineVolumeWeightedAverageTradingPrice" abstract="false" name="NumberOfDaysUsedToDetermineVolumeWeightedAverageTradingPrice" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="next_VolumeWeightedAverageTradingPriceThresholdMinimum" abstract="false" name="VolumeWeightedAverageTradingPriceThresholdMinimum" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:perShareItemType"/>
  <xs:element id="next_ServiceBasedRestrictedStockMemberMember" abstract="true" name="ServiceBasedRestrictedStockMemberMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_AccountsPayableAcquisitionOfPropertyPlantAndEquipment" abstract="false" name="AccountsPayableAcquisitionOfPropertyPlantAndEquipment" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="next_AccruedLiabilitiesAcquisitionOfPropertyPlantAndEquipment" abstract="false" name="AccruedLiabilitiesAcquisitionOfPropertyPlantAndEquipment" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="next_NoncashOrPartNoncashSettlementOfWarrantLiabilities" abstract="false" name="NoncashOrPartNoncashSettlementOfWarrantLiabilities" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="next_AccruedLiabilitiesDebtAndEquityIssuanceCosts" abstract="false" name="AccruedLiabilitiesDebtAndEquityIssuanceCosts" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="next_ReclassificationFromOtherNonCurrentAssetsToPropertyPlantAndEquipment" abstract="false" name="ReclassificationFromOtherNonCurrentAssetsToPropertyPlantAndEquipment" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="next_Train5LLCMember" abstract="true" name="Train5LLCMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_IndirectFinancingSubsidiariesMember" abstract="true" name="IndirectFinancingSubsidiariesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_Train5CreditAgreementMember" abstract="true" name="Train5CreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="next_CommittedEquityContributions" abstract="false" name="CommittedEquityContributions" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
</xs:schema>
