Attempt to de-reference a null object while serializing a custom object with nested objects to JSON in Apex

Question

Im running into an de-reference a null object error while trying to serialize a list of objects into JSON for a POST method. The error occurs when it tries to assign a value to line 277 (within the toJSON() function)shipstation_order.shipTo.street1 = cah_order[0].Address_1__c;. This must be due to me not creating instantiating the sub objects correctly. Any reference or advice would be greatly appreciated!

Here is my current code

public class TotalSales_to_Shipstation_Service implements Queueable,Database.AllowsCallouts{

    //************** Shipstation Classes ********************//
    
    public Order[] Shipstation_Orders;
    
    class Order {
        public String orderNumber;  
        public String orderKey; 
        public String orderDate;    
        public String paymentDate;  
        public String shipByDate;   
        public String orderStatus;  
        public Integer customerId;  
        public String customerUsername; 
        public String customerEmail;    
        cls_billTo billTo;
        cls_shipTo shipTo;
        cls_items[] items;
        public Double amountPaid;   
        public Integer taxAmount;   
        public Integer shippingAmount;
        public String customerNotes;
        public String internalNotes;
        public boolean gift;
        public String giftMessage;
        public String paymentMethod;
        public String requestedShippingService;
        public String carrierCode;
        public String serviceCode;
        public String packageCode;
        public String confirmation;
        public String shipDate;
        cls_weight weight;
        cls_dimensions dimensions;
        cls_insuranceOptions insuranceOptions;
        cls_internationalOptions internationalOptions;
        cls_advancedOptions advancedOptions;
        public Integer[] tagIds;
    }
    class cls_billTo {
        public String name;
        public String company;
        public String street1;
        public String street2;
        public String street3;
        public String city;
        public String state;
        public String postalCode;
        public String country;
        public String phone;
        public boolean residential;
    }
    class cls_shipTo {
        public String name;
        public String company;
        public String street1;
        public String street2;
        public String street3;
        public String city;
        public String state;
        public String postalCode;
        public String country;
        public String phone;
        public boolean residential;
    }
    class cls_items {
        public String lineItemKey;
        public String sku;
        public String name;
        public String imageUrl;
        cls_weight weight;
        public Integer quantity;
        public Double unitPrice;
        public Double taxAmount;
        public Integer shippingAmount;
        public String warehouseLocation;
        cls_options[] options;
        public Integer productId;
        public String fulfillmentSku;
        public boolean adjustment;
        public String upc;
    }

    class cls_weight {
        public Integer value;
        public String units;
    }
    class cls_options {
        public String name;
        public String value;
    }

    class cls_dimensions {
        public String units;
        public Integer length;
        public Integer width;
        public Integer height;
    }
    class cls_insuranceOptions {
        public String provider;
        public boolean insureShipment;
        public Integer insuredValue;
    }
    class cls_internationalOptions {
        public String contents;
        cls_customsItems customsItems;
    }

    class cls_customsItems {
        public String description;
        public Integer quantity;
        public Integer value;
        public Integer harmonizedTariffCode;
        public String countryOfOrigin;
    }
    class cls_advancedOptions {
        public Integer warehouseId;
        public boolean nonMachinable;
        public boolean saturdayDelivery;
        public boolean containsAlcohol;
        public boolean mergedOrSplit;
        public Integer[] mergedIds;
        public Integer parentId;
        public String storeId;
        public String customField1;
        public String customField2;
        public String customField3;
        public String source;
        public String billToParty;
        public String billToAccount;
        public String billToPostalCode;
        public String billToCountryCode;
    }
    
    //list of lists to hold grouped ts records with the same order id
    List<List<total_sales__c>> cah_orders_formatted = new List<List<total_sales__c>>();
    
    //query is for testing(not final query)
    total_sales__c[] ts_list =[
        SELECT  id,
                order_number__c,
                createdDate,
                Email__c,
                First_Name__c,
                Last_Name__c,
                Address_1__c,
                Address_2__c,
                City__c,
                State_Provence__c,
                Postal_Code__c,
                Country_Code__c
        FROM total_sales__c
        WHERE Source__c = 'sourcenamehere'
        ORDER BY createdDate DESC
        LIMIT 100
     ];
    
    //************** Methods ********************************//
    public void execute(QueueableContext context) {
        formatTS();
        Http http = new Http();
        HttpRequest request = new HttpRequest();
        request.setEndpoint(endpoint);
        request.setMethod('POST');
        request.setHeader('Content-Type', 'application/json;charset=UTF-8');
        
        String username = key;
        String password = secret;
        
        Blob headerValue = Blob.valueOf(username + ':' + password);
        String authorizationHeader = 'BASIC ' + EncodingUtil.base64Encode(headerValue);
        request.setHeader('Authorization', authorizationHeader);
        
        String jsonBody = toJSON();
        request.setBody(jsonBody);
        HttpResponse response = http.send(request);
        
        if (response.getStatusCode() != 201) {
            System.debug('The status code returned was not expected: ' +
        response.getStatusCode() + ' ' + response.getStatus());
        } else {
            System.debug(response.getBody());
        }

        
    }
    
    public void formatTS(){
        // create a list to track which order numbers are already in the formated cah list
        List<String> ordernumbers = new List<String>();
        
        //loop through each total sale record queried
        for(total_sales__c record : ts_list){
            //if the order number is not in the tracking list...
            if(!ordernumbers.contains(record.order_number__c)){
                //add it to the tracking list
                ordernumbers.add(record.order_number__c);
                //create a new total sales list to hold all of the records with the same order number
                List<total_sales__c> cah_order = new List<total_sales__c>();
                //add that ts record to the order
                cah_order.add(record);
                //then add that list to the list that will hold all of the grouped records by order number
                cah_orders_formatted.add(cah_order); 
             // if the order id is found...
            }else{ 
                //search the list of lists that contain the grouped records 
                for(List<total_sales__c> order : cah_orders_formatted){
                    //once the matching order id is found, add it to that list
                    if(record.order_number__c == order[0].order_number__c){
                        order.add(record);
                    }
                }
            }
            system.debug(cah_orders_formatted);
        }
               
        
    }
    
     public string ifblank(string in_string, string null_value){
            string null_txt = '';
            if ( null_value != null ){
                    null_txt = null_value.trim();
                }
            if (in_string == null ){
                return(null_txt);
            }
            string temp = in_string.trim();
            if ( temp.length() < 1 ){
                return(null_txt);
            }
            return(temp);
    }
    
    public String toJSON(){
        //parse through each list of grouped records
        for(List<total_sales__c> cah_order : cah_orders_formatted){
            //create a new order object for that list
            Order shipstation_order = new Order();
            
            //assign the various values to the shipstation object
            shipstation_order.orderNumber  = cah_order[0].order_number__c;
            //shipstation_order.orderKey(Not Needed)     
            shipstation_order.orderDate =  string.valueOf(cah_order[0].createdDate);
            //shipstation_order.paymentDate
            //shipstation_order.shipByDate 
            shipstation_order.orderStatus = 'awaiting_shipment'; //(String- "awaiting_payment", "awaiting_shipment", "shipped", "on_hold", "cancelled")
            //shipstation_order.customerId(Read Only)
            shipstation_order.customerUsername = cah_order[0].Email__c;
            shipstation_order.customerEmail = cah_order[0].Email__c;
            //
            //shipstation_order.billTo.name(String- First and last)
            //shipstation_order.billTo.company(String)
            //shipstation_order.billTo.street1(String)
            //shipstation_order.billTo.street2
            //shipstation_order.billTo.street3
            //shipstation_order.billTo.city(String)
            //shipstation_order.billTo.state(String)
            //shipstation_order.billTo.postalCode(String)
            //shipstation_order.billTo.country(String country_code__c)
            //shipstation_order.billTo.phone(String)
            //shipstation_order.billTo.residential(Boolean)
            //
            //shipstation_order.shipTo.name = cah_order[0].First_Name__c + cah_order[0].Last_Name__c;
            //shipstation_order.shipTo.company 
            shipstation_order.shipTo.street1 = cah_order[0].Address_1__c;
            shipstation_order.shipTo.street2 = cah_order[0].Address_2__c;
            //shipstation_order.shipTo.street3
            shipstation_order.shipTo.city = cah_order[0].City__c;
            shipstation_order.shipTo.state = cah_order[0].State_Provence__c;
            shipstation_order.shipTo.postalCode = cah_order[0].Postal_Code__c;
            shipstation_order.shipTo.country = cah_order[0].Country_Code__c;
            //shipstation_order.shipTo.phone
            //shipstation_order.shipTo.residential
            //
            //shipstation_order.amountPaid(number)
            //shipstation_order.taxAmount(number)
            //shipstation_order.shippingAmount(number)
            //shipstation_order.customerNotes(String)
            //shipstation_order.internalNotes(String)
            //shipstation_order.gift(Boolean)
            //shipstation_order.giftMessage(String)
            //shipstation_order.paymentMethod(String)
            //shipstation_order.requestedShippingService(???)
            //shipstation_order.carrierCode(String)
            //shipstation_order.serviceCode(String)
            //shipstation_order.packageCode(String)
            //shipstation_order.confirmation(String)
            //shipstation_order.shipDate(string)
            //
            //shipstation_order.weight.value(Number)
            //shipstation_order.weight.units(String)
            //
            //shipstation_order.dimensions.units(String)
            //shipstation_order.dimensions.lenght(Number)
            //shipstation_order.dimensions.width(Number)
            //shipstation_order.dimensions.height(Number)
            //
            //shipstation_order.insuranceOptions.provider(String-  "shipsurance", "carrier", or "provider")
            //shipstation_order.insuranceOptions.insureShipment(Boolean)
            //shipstation_order.insuranceOptions.insuredValue(Number)
            //
            //shipstation_order.internationalOptions.contents(String- "merchandise", "documents", "gift", "returned_goods", or "sample")
            //shipstation_order.internationalOptions.customsItems.description(String)
            //shipstation_order.internationalOptions.customsItems.quantity(Number)
            //shipstation_order.internationalOptions.customsItems.value(Number in USD)
            //shipstation_order.internationalOptions.customsItems.harmonizedTariffCode(String)
            //shipstation_order.internationalOptions.customsItems.countryOfOrigin(String Country_code__c)
            //
            //shipstation_order.advancedOptions.warehouseId
            //shipstation_order.advancedOptions.nonMachinable
            //shipstation_order.advancedOptions.saturdayDelivery
            //shipstation_order.advancedOptions.containsAlcohol
            //shipstation_order.advancedOptions.mergedOrSplit
            //shipstation_order.advancedOptions.mergedIds
            //shipstation_order.advancedOptions.parentId
            shipstation_order.advancedOptions.storeId = storeID;
            //shipstation_order.advancedOptions.customField1
            //shipstation_order.advancedOptions.customField2
            //shipstation_order.advancedOptions.customField3
            //shipstation_order.advancedOptions.source
            //shipstation_order.advancedOptions.billToParty
            //shipstation_order.advancedOptions.billToAccount
            //shipstation_order.advancedOptions.billToPostalCode
            //shipstation_order.advancedOptions.billToCountryCode
            //
            //shipstation_order.tagIds
            //
            for(total_sales__c record: cah_order){
                cls_items item = new cls_items();
                //item.lineItemKey
                //item.sku
                //item.name
                //item.imageUrl
                //item.weight.value
                //item.weight.units
                //item.quantity
                //item.unitPrice
                //item.taxAmount
                //item.shippingAmount
                //item.warehouseLocation
                //item.options.name
                //item.options.value
                //item.productId
                //item.fulfillmentSku
                //item.adjustment
                //item.upc
                shipstation_order.items.add(item);
            }
            Shipstation_Orders.add(shipstation_order);
        }
        String jsonBody = JSON.serialize(Shipstation_Orders);
        return jsonBody;
    }
    

    
}