Why do my cbot stop runing ?

I writed some code something like that

 protected override void OnTick()
        {
            var longPosition = Positions.Find(label, Symbol, TradeType.Buy);
            var shortPosition = Positions.Find(label, Symbol, TradeType.Sell);

            var SoLenh = Positions.Count(x => x.SymbolCode == Symbol.Code);

         }

if (RSI > 65)
            {
                if (SoLenh < 3)
                    ExecuteMarketOrder(TradeType.Sell, Symbol, VolumeInUnits, label);
                if (longPosition != null)
                    ClosePosition(longPosition);
            }
else if (RSI < 35)
            {
                if (SoLenh < 3)
                    ExecuteMarketOrder(TradeType.Buy, Symbol, VolumeInUnits, label);
                if (shortPosition != null)
                    ClosePosition(shortPosition);
            }
else if (RSI < 30 && shortPosition != null)
                ClosePosition(shortPosition);
else if (RSI > 70 && longPosition != null)
                ClosePosition(longPosition);

It can runing without any errors but after few time it stop and the log showed:

27/07/2018 01:09:42.191Crashed in Positions.Opened with NullReferenceException: Object reference not set to an instance of an object.

27/07/2018 01:09:42.238cBot "New cBot" was stopped for EURCHF, m2.
 

Please help I cannot know when it stop runing to start it again!

Thanks so much!