Crashed in Positions.Opened Null reference, only during backest.

Hi,

I have a problem where during backtest I get a null reference exception error when using OnPositionedOpened event:

The strange thing is during live testing the code works perfectly, just during backtest I get an null reference error:

The error is:
"Crashed in Positions.Opened with NullReferenceException: Object reference not set to an instance of an object."

It occurs when i try to open another position from within the onPosition Open event:

  

  protected override void OnStart()
        {
            Positions.Opened += Positions_Opened;
            Positions.Closed += PositionsClosed;

        }

private void Positions_Opened(PositionOpenedEventArgs args)
        {

             //Code to open new positions

        }

private void PositionsClosed(PositionClosedEventArgs args)
        {

           //code to close all positions

         }

What could be the cause?