Intermediate-UserStrategy-class-with-common-code

I try to add some common functionality to a trading strategy. As an example let me compose a daily soup:

CODE:
using System;
using System.Collections.Generic;
using WealthLab.Backtest;
using WealthLab.Core;
namespace WealthScript4 
{
    public class MyStrategy : UserStrategyBase
    {
        public override void PreExecute(DateTime dt, List<BarHistory> participants)
        {
         dailySoup = 0.0; // Init Daily Soup
        }

        public override void Execute(BarHistory bars, int idx)
      {
         double percentage = 2.0;
         foreach (var pos in OpenPositions) ClosePosition(pos, OrderType.Market);
           double limit = bars.Close[idx] * (1.0 - percentage / 100.0);
         Transaction tr = PlaceTrade(bars, TransactionType.Buy, OrderType.Limit, limit);

         dailySoup += limit; // Collect Daily Soup
        }

        public override void PostExecute(DateTime dt, List<BarHistory> participants)
        {
         // Finish Daily Soup
         WLHost.Instance.AddLogItem(StrategyName, "Daily soup: " + dailySoup.ToString("f2"),
            WLColor.Blue);
        }

        private double dailySoup;
    }
}

(This is OneNight with daily soup)

Because I want to use the very same mechanism in several strategies I move it to a common base class called UserStrartegyWithSoup like so:

CODE:
using System;
using System.Collections.Generic;
using WealthLab.Backtest;
using WealthLab.Core;
namespace WealthScript3 
{
   // intermeddiate base class with soup
   public abstract class UserStrategyWithSoup : UserStrategyBase
   {
      public override void PreExecute(DateTime dt, List<BarHistory> participants)
      {
         dailySoup = 0.0; // Init Daily Soup
      }
      
      public override abstract void Execute(BarHistory bars, int idx);

      public override void PostExecute(DateTime dt, List<BarHistory> participants)
      {
         // Finish Daily Soup
         WLHost.Instance.AddLogItem(StrategyName, "Daily soup: " + dailySoup.ToString("f2"),
            WLColor.Blue);
      }

      protected double dailySoup;
   }
   
   
    public class MyStrategy : UserStrategyWithSoup
    {
        public override void Execute(BarHistory bars, int idx)
      {
         double percentage = 2.0;
         foreach (var pos in OpenPositions) ClosePosition(pos, OrderType.Market);
           double limit = bars.Close[idx] * (1.0 - percentage / 100.0);
         Transaction tr = PlaceTrade(bars, TransactionType.Buy, OrderType.Limit, limit);

         dailySoup += limit; // Collect Daily Soup
        }
    }
}

I think this is technically correct but it does not even compile in WL8. I get this exception:

CODE:
System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Object.GetType()
   at WealthLab8.designerCode.Compile()
   at WealthLab8.designerCode.RevertDescriptor(Object ident, RoutedEventArgs vis)
...

How can I compose my daily soup?