System.NullReferenceException in Kruskal's algortihm

Question

I'm trying to implement Kruskal's algortihm in C# and I got such

NullReferenceException: subsets[] was null

Here is my class Graph and Kruskal's algortihm with some needed methods

using System;
using System.Collections.Generic;
using System.Text;
using System.Linq;

namespace Application
{
    public class Graph
    {
        public List<Vertex> Vertexes = new List<Vertex>();
        public List<Edge> Edges = new List<Edge>();

        public int VertexCount => Vertexes.Count;
        public int EdgesCount => Edges.Count;

        public void Add(Vertex vertex)
        {
            Vertexes.Add(vertex);
        }
        public void Add(Vertex from, Vertex to, int weight)
        {
            var edge = new Edge(from, to, weight);
            Edges.Add(edge);
        }
       
        public void Kruskal(Graph graph)
        {
            Edge[] edges = Edges.ToArray();
            int verticesCount = graph.VertexCount;            
            Edge[] result = new Edge[verticesCount];
            int i = 0;
            int e = 0;

            Array.Sort(edges, delegate (Edge a, Edge b)
            {
                return a.Weight.CompareTo(b.Weight);
            });

            //exception is here
            Subset[] subsets = new Subset[verticesCount];
            for (int v = 0; v < verticesCount; ++v)
            {
                subsets[v].Parent = v;
                subsets[v].Rank = 0;
            }

            while (e < verticesCount - 1)
            {
                Edge nextEdge = edges[i++];
                int x = Find(subsets, nextEdge.From.Number);
                int y = Find(subsets, nextEdge.To.Number);

                if (x != y)
                {
                    result[e++] = nextEdge;
                    Union(subsets, x, y);
                }
            }

            Print(result, e);
        }

        private int Find(Subset[] subsets, int i)
        {
            if (subsets[i].Parent != i)
                subsets[i].Parent = Find(subsets, subsets[i].Parent);

            return subsets[i].Parent;
        }

        private void Union(Subset[] subsets, int x, int y)
        {
            int xroot = Find(subsets, x);
            int yroot = Find(subsets, y);

            if (subsets[xroot].Rank < subsets[yroot].Rank)
                subsets[xroot].Parent = yroot;
            else if (subsets[xroot].Rank > subsets[yroot].Rank)
                subsets[yroot].Parent = xroot;
            else
            {
                subsets[yroot].Parent = xroot;
                ++subsets[xroot].Rank;
            }
        }

        private static void Print(Edge[] result, int e)
        {
            for (int i = 0; i < e; ++i)
                Console.WriteLine("{0} -- {1} == {2}", result[i].From.Number, result[i].To.Number, result[i].Weight);
        }
    }
}

Here is my Subset class

using System;
using System.Collections.Generic;
using System.Text;

namespace Application
{
    class Subset
    {
        public int Parent;
        public int Rank;
    }
}

I can't actually understand, why my array is empty and how to fix this issue. If you know something, please, write about it