问为什么我在尝试给属性数组设置值时会得到NullReferenceException？

EN

fac = new Facility();
List<FacilityCell> gens = new List<FacilityCell>();
for (int i = 0; i < 6; i++)
{
    gens.Add(new FacilityCell(i.ToString(), i));
}

fac.Genes = gens.ToArray();
fac.Cells = gens.ToArray();

float[] dems = new float[3];
dems[0] = 300;
dems[1] = 60;
dems[2] = 160;
fac.Demands = dems;

FacilityCell[][] fl = new FacilityCell[3][];
fl[0] = new FacilityCell[] { 
    fac.Cells[0],
    fac.Cells[2],
    fac.Cells[4],
    fac.Cells[1],
    fac.Cells[3],
    fac.Cells[5] };
fl[1] = new FacilityCell[] {
    fac.Cells[2], 
    fac.Cells[4], 
    fac.Cells[1], 
    fac.Cells[5], 
    fac.Cells[3], 
    fac.Cells[4] };
fl[2] = new FacilityCell[] { 
    fac.Cells[1], 
    fac.Cells[0], 
    fac.Cells[4], 
    fac.Cells[2], 
    fac.Cells[3], 
    fac.Cells[5] };
fac.Flows = fl;

public class Facility : IChromosome
{
    public Facility()
    {

    }
    public Facility(FacilityCell[] cells)
    {
        this.cells = cells;
        flows = null;
        demands = null;
        for (int i = 0; i < cells.Length; i++)
        {
            cells[i].Order = i;
        }
    }

    private IGene[] cells;
    private float[] demands;
    private FacilityCell[][] flows;

    public FacilityCell[][] Flows
    {
        get { return flows; }
        set { flows = value; }
    }
    public FacilityCell[] Cells
    {
        get
        {
            return cells as FacilityCell[];
        }
        set
        {
            cells = value;
        }
    }

    public float[] Demands
    {
        get { return demands; }
        set { demands = value; }
    }

    public float FitValue
    {
        get
        {
            float total = 0;

            //I AM GETTING ERROR IN THIS LINE OF CODE, THE FOR LOOP
            //It throws NullReferenceException for both this.Demands and this.Flows

            for (int i = 0; i < flows.Length; i++)
            {
                for (int j = 0; j < flows[i].Length - 1; j++)
                {
                    int dist = Math.Abs(flows[i][j + 1].Order - flows[i][j].Order);
                    float totflow = dist * demands[i];
                    total += totflow;
                }
            }
            return total;
        }
    }

    public IGene[] Genes
    {
        get
        {
            return cells;
        }
        set
        {
            cells = value;
        }
    }
}