Object reference not set to an instance of an object Error in create json body

@jeewanishalika20 I am not sure what are you trying to do. If you are meaning to create SO from PO, there are better and more reliable options available. Anyways I did some re-write on your code to generate the json code. I tested it works, removes the PO from hold and based on my Status Diagram sets it to Email Purchase Order. Please note as I didn’t now how you are defining your variables like “order” I replaced them with Base Views so you might want to change them. Here is the revised code:

using System.Collections;
using System.Collections.Generic;
using System.Text.Json;
using PX.Data;
using PX.Objects.PO;
using PX.Objects.SO;
using PX.Web.UI;

namespace HCL.Custom.Misc
{
    public class HCLPOOrderEntry_Misc : PXGraphExtension<POOrderEntry>
    {
        public static bool IsActive() => true;

        public delegate IEnumerable ReleaseFromHoldDelegate(PXAdapter adapter);
        [PXOverride]
        [PXButton]
        public virtual IEnumerable ReleaseFromHold(PXAdapter adapter, ReleaseFromHoldDelegate baseMethod)
        {
            RootObject obj = new RootObject();
            obj.OrderType = Base.Document.Current.OrderType;
            obj.OrderNbr = Base.Document.Current.OrderNbr;
            obj.PromisedOn = Base.Document.Current.ExpectedDate?.ToString();
            obj.Status = Base.Document.Current.Status;

            foreach (PXResult<POLine> result in Base.Transactions.Select())
            {

                POLine line = result;
                Details detail = new Details();

                detail.Amount = line.CuryExtCost?.ToString();
                detail.CustomerOrderNbr = SOOrder.PK.Find(Base, "SO", line.SOOrderNbr)?.CustomerOrderNbr;
                detail.DiscountAmount = line.CuryDiscAmt?.ToString();
                detail.DiscountPercent = line.DiscPct?.ToString();

                if (obj.Details == null)
                    obj.Details = new List<Details>();

                obj.Details.Add(detail);
            }


            var options = new JsonSerializerOptions { WriteIndented = true };
            var json = JsonSerializer.Serialize(obj, options);

            PXTrace.WriteInformation(json);

            return baseMethod.Invoke(adapter);
        }

        public class RootObject
        {
            public string OrderType { get; set; }
            public string OrderNbr { get; set; }
            public string PromisedOn { get; set; }
            public string Status { get; set; }
            public List<Details> Details { get; set; }

        }

        public class Details
        {
            public string Amount { get; set; }
            public string CustomerOrderNbr { get; set; }
            public string DiscountAmount { get; set; }
            public string DiscountPercent { get; set; }
        }
    }
}

"{\r\n  \"OrderType\": \"SB\",\r\n  \"OrderNbr\": \"PO000115\",\r\n  \"PromisedOn\": \"06/30/2022 12:00:00 AM\",\r\n  \"Status\": \"H\",\r\n  \"Details\": [\r\n    {\r\n      \"Amount\": \"36538.0000\",\r\n      \"CustomerOrderNbr\": null,\r\n      \"DiscountAmount\": \"0.0000\",\r\n      \"DiscountPercent\": \"0.000000\"\r\n    }\r\n  ]\r\n}"