why so null

using System;
using System.Linq;
using System.Threading.Tasks;
using EmiterPlacer.WavePropagation.Core;

namespace EmiterPlacer.Placement.GeneticOptimizer.Evaluators
{
    public class ThresholdCutoffEvaluator: IPopulationEvaluator
    {
        private Env2D[] _enviromentPool;
        public float SignalLevelThreshold { get; set; }
        public void EvaluatePopulation(Placement[] population, Env2D environment, int maxEmiters)
        {
            this.InitEnvPool(environment);
            if (this.SignalLevelThreshold > 1)
            {
                throw new Exception("Signal level over 1 cannot be possibly reached!");
            }
            var pl = 0;
            var groupSize = population.Length/100 + 1;
            var tasks = new Task[100];
            for (int i = 0; i < 99; i++)
            {
                tasks[i] = Task.Factory.StartNew(() => this.EvaluatePlacements(population.Skip(i*groupSize).Take(groupSize).ToArray(), _enviromentPool[i], maxEmiters));
                Console.WriteLine("Evaluation, task: " + i);
            }
            tasks[99] = Task.Factory.StartNew(() => this.EvaluatePlacements(population.Skip(99 * groupSize).Take(population.Length - 99 * groupSize).ToArray(), _enviromentPool[99], maxEmiters));
            Console.WriteLine("Evaluation, task: " + 99);
            Task.WaitAll(tasks);

        }

        private void InitEnvPool(Env2D environment)
        {
            _enviromentPool = new Env2D[100];
            for(var i = 0 ; i < _enviromentPool.Length; i++)
            {
                _enviromentPool[i] = new Env2D(environment);
            }
        }

        private void EvaluatePlacements(Placement[] population, Env2D environment, int maxEmiters)
        {
            foreach (var placement in population)
            {
                EvaluatePlacement(placement,environment,maxEmiters);
            }
        }

        private void EvaluatePlacement(Placement placement, Env2D environment, int maxEmiters)
        {
            if (placement.Fitness == null) //do not recalculate if nothing changed
            {
                var elementsInReach = environment.PercentageCoveredForEmitterPositions(SignalLevelThreshold, placement.EmiterPositions);
                var numOfEmiters = placement.NumOfEmiters;
                placement.Fitness = elementsInReach;
                placement.Fitness += Math.Abs(elementsInReach - 1) < 0.0001 ? (float)(maxEmiters - numOfEmiters) / maxEmiters : 0;
            }
        }
    }
}