Hello,

I'm trying to re-purpose the code of the ATR indicator to make another indicator, but when I try to use the new indicator on a sample algorithm, the error is null reference in the ComputeNextValue method. Would appreciate any assistance!

using System;
using QuantConnect.Data.Market;

namespace QuantConnect.Indicators
{
/// <summary>
/// The AverageTrueRange indicator is a measure of volatility introduced by Welles Wilder in his
/// book: New Concepts in Technical Trading Systems. This indicator computes the TrueRange and then
/// smoothes the TrueRange over a given period.
///
/// TrueRange is defined as the maximum of the following:
/// High - Low
/// ABS(High - PreviousClose)
/// ABS(Low - PreviousClose)
/// atrDerivative
/// </summary>
public class atrDerivative : BarIndicator, IIndicatorWarmUpPeriodProvider
{
private IBaseDataBar _previous;

private decimal atrLong;

private readonly decimal _multipleL;

/// <summary>This indicator is used to smooth the TrueRange computation</summary>
/// <remarks>This is not exposed publicly since it is the same value as this indicator, meaning
/// that this '_smoother' computers the ATR directly, so exposing it publicly would be duplication</remarks>
///private readonly IndicatorBase<IndicatorDataPoint> _smoother;

private IndicatorBase<IndicatorDataPoint> _smootherL;

private readonly IndicatorBase<IndicatorDataPoint> _atrDerivative;

/// <summary>
/// Gets the true range which is the more volatile calculation to be smoothed by this indicator
/// </summary>
public IndicatorBase<IBaseDataBar> TrueRange { get; } //doesn't necessarily need to be public

/// <summary>
/// Gets a flag indicating when this indicator is ready and fully initialized
/// </summary>
public override bool IsReady => _smootherL.IsReady;


/// <summary>
/// Required period, in data points, for the indicator to be ready and fully initialized.
/// </summary>
public int WarmUpPeriod { get; }

/// <summary>
/// Creates a new atrDerivative indicator using the specified period, multiple, and moving average types
/// </summary>
/// <param name="name">The name of this indicator</param>
/// <param name="periodL">The smoothing period used to smooth the true range values for long position</param>
/// <param name="multipleL">The the multiple for ATR of long position</param>
/// <param name="movingAverageTypeL">The type of smoothing used to smooth the true range values for long positions</param>
public atrDerivative(string name, int periodL = 20, decimal multipleL = 3m, MovingAverageType movingAverageTypeL = MovingAverageType.Wilders)
: base(name)
{
WarmUpPeriod = periodL;
_multipleL = multipleL;
_smootherL = movingAverageTypeL.AsIndicator($"{name}_{movingAverageTypeL}", periodL);

TrueRange = new FunctionalIndicator<IBaseDataBar>(name + "_TrueRange", currentBar =>
{
// in our ComputeNextValue function we'll just call the ComputeTrueRange
var nextValue = ComputeTrueRange(_previous, currentBar);
_previous = currentBar;
return nextValue;
} // in our IsReady function we just need at least one sample
, trueRangeIndicator => trueRangeIndicator.Samples >= 1
);
}

/// <summary>
/// Creates a new atrDerivative indicator using the specified period, multiple, and moving average types
/// </summary>
/// <param name="periodL">The smoothing period used to smooth the true range values for long position</param>
/// <param name="multipleL">The the multiple for ATR of long position</param>
/// <param name="movingAverageTypeL">The type of smoothing used to smooth the true range values for long positions</param>
public atrDerivative(int periodL = 20, decimal multipleL = 3m, MovingAverageType movingAverageTypeL = MovingAverageType.Wilders)
: this($"ATRD({periodL},{multipleL},{movingAverageTypeL})", periodL, multipleL, movingAverageTypeL)
{
}

/// <summary>
/// Computes the TrueRange from the current and previous trade bars
///
/// TrueRange is defined as the maximum of the following:
/// High - Low
/// ABS(High - PreviousClose)
/// ABS(Low - PreviousClose)
/// </summary>
/// <param name="previous">The previous trade bar</param>
/// <param name="current">The current trade bar</param>
/// <returns>The true range</returns>
public static decimal ComputeTrueRange(IBaseDataBar previous, IBaseDataBar current)
{
var range1 = current.High - current.Low;
if (previous == null)
{
return range1;
}

var range2 = Math.Abs(current.High - previous.Close);
var range3 = Math.Abs(current.Low - previous.Close);

return Math.Max(range1, Math.Max(range2, range3));
}

/// <summary>
/// Computes the next value of this indicator from the given state
/// </summary>
/// <param name="input">The input given to the indicator</param>
/// <returns>A new value for this indicator</returns>
protected override decimal ComputeNextValue(IBaseDataBar input)
{
TrueRange.Update(input);
_smootherL.Update(input.Time, TrueRange);
_atrDerivative.Update(input.Time, input.Close - (_multipleL * _smootherL));
return _atrDerivative.Current.Value;

}

public override void Reset()
{
_previous = null;
_smootherL.Reset();
TrueRange.Reset();
_atrDerivative.Reset();
base.Reset();
}
}
}