Hi, Would anyone know what the issue is here. I am getting an error: Update failed. First exception on row 0; first error: MISSING_ARGUMENT, Id not specified in an update call. Error is in expression '{!multiInvoice}' in component <apex:commandButton> in page womultiinvoice: Class.woMultiInvoice.multiInvoice: line 138, column 1

The apex code is below:

public class woMultiInvoice {
    private ApexPages.StandardSetController standardSetController;
    public list<workOrder> woListSelected{get; set;}
    public string PONumber{get;set;}
    
    public woMultiInvoice(ApexPages.StandardSetController standardSetController){
        this.standardSetController = standardSetController;
        List<workOrder> selectedListViewRecords = (List<workOrder>) standardSetController.getSelected();
        woListSelected = [select id, locationId, AccountId, WorkOrderNumber, Contract__c, AgreedLabourRate__c, PartsQtyCustomerCharge__c, CustomerReport__c,
                          FirstHourCharge__c, FirstHourRate__c, GeneralLabour__c, HoursCustomerQty__c, TotalLabourPrice__c, Description, Make__c, Model__c, SerialNo__c,
                          (select id, WorkOrderId, Invoice__c, QuantityConsumed, UnitPrice, Product2.Name, Description, TotalPrice__c
                          from ProductsConsumed where invoice__c=null)
                          from WorkOrder where id in:selectedListViewRecords];
    }
    
    public PageReference multiInvoice(){
        
        //place the woListSelected back into the VF page for the Invoicer to review the records
        
        
        //system.debug(selectedListViewRecords);
        list<workOrder> woList;
        set<id> AccountIdSet = new set<id>();
        map<Account, list<workOrder>> invoiceMap = new map <Account, list<workOrder>>();
        
        for(workOrder wo: woListSelected){
            AccountIdSet.add(wo.AccountId);
        }
        system.debug('Number of Contracts in Set: '+AccountIdSet.size());
        
        for(Account acc: [select id from Account where id in:AccountIdSet]){
            woList = new list<WorkOrder>();
            for(workOrder wo: woListSelected){
                if( wo.AccountId ==acc.id)woList.add(wo);
            }
            invoiceMap.put(acc, woList);
        }
        
        
        list<workOrder> woToInvoice = new list<workOrder>();
        list<ProductConsumed> partsList = new list<ProductConsumed>();
        list<timeSheetEntry> labourList = new list<timeSheetEntry>();
        list<InvoiceLineItem__c> invItmList = new list<InvoiceLineItem__c>();
        invoice__c inv = new invoice__c();
        
        for(Account acc:invoiceMap.keyset()){
            woToInvoice = invoiceMap.get(acc);
            partsList = [select id, WorkOrderId, Invoice__c, QuantityConsumed, UnitPrice, Product2.Name, Description, TotalPrice__c
                         from ProductConsumed where invoice__c=null and workOrderId in:woToInvoice];
            
            labourList = [select id, WorkOrderId, Invoice__c, TotalTime__c, NormalTime__c, Overtime__c, OverTimeCharge__c, FirstHour__c, StandardTimeCharge__c
                          from timeSheetEntry where invoice__c=null and workOrderId in:woToInvoice];
            
            inv = new invoice__c(Type__c ='Service Invoice', Cust_Order_No__c = PONumber, Customer_Account_Name__c = acc.id, VAT_Code__c='G',
                                 Invoice_To__c = acc.id);
            insert inv;
            system.debug(inv);
            
            list<InvoiceLineItem__c>partLinesList = new list<InvoiceLineItem__c>();
            list<InvoiceLineItem__c>LabourLinesList = new list<InvoiceLineItem__c>();
            
            for(WorkOrder wo: woToInvoice){
                inv.Contract__c = wo.Contract__c;
                system.debug(wo.ProductsConsumed);
                if(!partsList.isEmpty()){
                    
                    InvoiceLineItem__c invItmParts;
                    for(ProductConsumed pc : partsList){
                        if(pc.WorkOrderId == wo.id){
                            invItmParts = new InvoiceLineItem__c(Name = wo.WorkOrderNumber+' - Parts', Invoice__c = inv.id);
                            invItmParts.Description__c = pc.Product2.Name+' - '+pc.Description;
                            invItmParts.Price__c = pc.UnitPrice;
                            invItmParts.Quantity__c = pc.QuantityConsumed;
                            pc.Invoice__c = inv.id;
                            partLinesList.add(invItmParts);
                            //update wo.ProductsConsumed; 
                        }
                    }
                }
                system.debug(wo.TimeSheetEntries);
                if(!labourList.isEmpty()){
                    InvoiceLineItem__c invItmLabour;
                    for(TimesheetEntry tse: labourList){
                        if(tse.WorkOrderId == wo.id){
                            if(tse.FirstHour__c >0){
                                invItmLabour = new InvoiceLineItem__c(Name = wo.WorkOrderNumber+' - Labour', Invoice__c = inv.id);
                                invItmLabour.Description__c = 'Labour - First Hour';
                                invItmLabour.Quantity__c = tse.FirstHour__c;
                                invItmLabour.Price__c = wo.FirstHourRate__c;
                                LabourLinesList.add(invItmLabour);                 
                            }
                            if(tse.StandardTimeCharge__c >0){
                                invItmLabour = new InvoiceLineItem__c(Name = wo.WorkOrderNumber+' - Labour', Invoice__c = inv.id);
                                invItmLabour.Description__c = 'Labour';
                                invItmLabour.Quantity__c = tse.StandardTimeCharge__c;
                                invItmLabour.Price__c = wo.AgreedLabourRate__c;
                                LabourLinesList.add(invItmLabour);
                            }
                            if(tse.OverTimeCharge__c >0){
                                invItmLabour = new InvoiceLineItem__c(Name = wo.WorkOrderNumber+' - Labour', Invoice__c = inv.id);
                                invItmLabour.Description__c = 'Labour - OOH';
                                invItmLabour.Quantity__c = tse.OverTimeCharge__c;
                                invItmLabour.Price__c = wo.OOHRate__c;
                                LabourLinesList.add(invItmLabour);
                            }
                            tse.Invoice__c =  inv.id;
                        }
                        //update wo.TimeSheetEntries;
                    }
                    
                }
                
                wo.InvoiceStatus__c = 'Invoiced';
                wo.Invoice__c = inv.id;

                
            }
            insert partLinesList;
            insert LabourLinesList;
            update partsList;
            update labourList;
            
            map<id, ServiceAppointment> SAMap = new map<id,ServiceAppointment>([select id, ParentRecordId from ServiceAppointment where ParentRecordId in:woToInvoice]);
            list<ServiceReport> sReportList = [select id, ParentId, CONTENTVERSIONDOCUMENTID, CONTENTVERSIONDOCUMENT.ContentDocumentId from ServiceReport where ParentId in:SAMap.keySet()];
            list<contentDocumentLink> cdlList = new list<contentDocumentLink>();
            for(serviceReport sr: SReportList){
                ServiceAppointment sa = SAMap.get(sr.ParentId);
                contentDocumentLink cdl = new contentDocumentLink();
                cdl.LinkedEntityId = inv.id;
                cdl.ContentDocumentId = sr.CONTENTVERSIONDOCUMENT.ContentDocumentId;
                cdlList.add(cdl);                
                
            }
            insert cdlList;
            
        }

        update woListSelected;
        update inv;
        system.debug(inv.id);
        
        PageReference reloadPage = new PageReference ('/lightning/r/Invoice__c/'+inv.id+'/view');
        system.debug(reloadPage);
        reloadPage.setRedirect(true);
        return reloadPage;
    
    }