Find historical trade using Contains() on Label

Hi,

I want to find some historical trades that meet criteria such as a certain string contained in the label. Is this correct?

protected override void OnTick()
{
            var Str = "ABCDEF";

            ExecuteMarketOrder(TradeType.Sell, SymbolName, Symbol.VolumeInUnitsMin, Str);
            ClosePosition(Positions.Find(Str));

            var X = History.Where(T => T.Label.Contains("ABC")).Count();
            Print(X);
}

This code return error during Count() function…

30/10/2023 15:49:17.632 | Crashed in OnTick with NullReferenceException: Object reference not set to an instance of an object.

Please help me do it correctly.

Thanks.