LastPosition-stopped-working

CODE:

using WealthLab.Backtest;
using System;
using WealthLab.Core;
using WealthLab.Indicators;
using WealthLab.ChartWPF;
using System.Drawing;
using System.Collections.Generic;

namespace WealthScript5 
{
   public class MyStrategy : UserStrategyBase
   {
      //create indicators and other objects here, this is executed prior to the main trading loop
      public override void Initialize(BarHistory bars)
      {
         cmo = new CMO(bars.Close, 14);
         PlotIndicator(cmo);
      }

      //execute the strategy rules here, this is executed once for each bar in the backtest history
      public override void Execute(BarHistory bars, int idx)
      {
         
         if (LastPosition == null)
         {
            WriteToDebugLog(bars.DateTimes[idx].ToShortDateString());
            
            if (cmo.CrossesOver(cmo.OversoldLevel, idx))
               PlaceTrade(bars, TransactionType.Buy, OrderType.Market);
         }
         else
         {
            Position p = LastPosition;
            
            //exit after 3 days
            if (idx + 1 - p.EntryBar >= 3)
               ClosePosition(p, OrderType.Market);
         }
      }

      //declare private variables below
      private CMO cmo;
   }
}