Почему вылетает с ошибкой NullReferenceException?

unit GraphOptimized;
 
uses GraphABC;
 
const
  Radius = 5;
 
type
  ///Location object
  Location = class
  private 
    dx, dy: integer;
    
    function XpositionRead()  := dx;
    
    function YpositionRead()  := dy;
    
    procedure XpositionWrite(v: integer);
    begin
      dx := v;
    end;
    
    procedure YpositionWrite(v: integer);
    begin
      dy := v;
    end;
  
  public 
    constructor(x, y: integer);
    begin
      dx := x;dy := y;
    end;
    
    ///X
    property Xposition: integer read XpositionRead write XpositionWrite;
    ///Y
    property Yposition: integer read YpositionRead write YpositionWrite;
  end;
  
  
  ///Graph's vertex
  GraphVertex = class
  private 
    p: Location;
    n: string;
    marked: boolean;
    Lev: integer;
    col: Color;
    function PositionRead()  := p;
    
    procedure PositionWrite(v: Location);
    begin
      p := v;
    end;
    
    function NameRead()  := n;
    
    procedure NameWrite(v: string);
    begin
      n := v;
    end;
    
    function IsMarkedRead()  := marked;
    
    procedure IsMarkedWrite(v: boolean);
    begin
      marked := v;
    end;
    
    function LevelRead()  := Lev;
    
    procedure LevelWrite(v: integer);
    begin
      Lev := v;
    end;
    
    function ColorValueRead()  := col;
    
    procedure ColorValueWrite(v: color);
    begin
      col := v;
    end;
  
  public 
    ///List of linked vertices
    Vertices: List<GraphVertex>;
    
    constructor(x, y: integer);
    begin
      p := new Location(x, y);marked := false;
    end;
    
    ///Position of the vertex
    property Position: Location read PositionRead write PositionWrite;
    ///Vertex'es name
    property Name: string read NameRead write NameWrite;
    ///The vertex is marked
    property IsMarked: boolean read IsMarkedRead write IsMarkedWrite;
    ///Level value (for DFS and other seaching algorithms)
    property Level: integer read LevelRead write LevelWrite;
    ///Color of the vertex
    property ColorValue: color read ColorValueRead write ColorValueWrite;
    
    ///Draw free vertex
    procedure Draw(v: GraphVertex);
    begin
      SetPenColor(clBlack);
      SetPenWidth(1);
      SetBrushColor(ColorValue);
      FillCircle(v.Position.Xposition, v.Position.Yposition, Radius);
      DrawCircle(v.Position.Xposition, v.Position.Yposition, Radius);
      SetBrushColor(ARGB(0, 0, 0, 0));
      if IsMarked then
        TextOut(v.Position.Xposition + 10, v.Position.Yposition + 10, v.Name + ' X [' + IntToStr(Lev) + ']')
      else
        TextOut(v.Position.Xposition + 10, v.Position.Yposition + 10, v.Name + ' [' + IntToStr(Lev) + ']');
    end;
    
    ///Draw all vertices, those are liked to the current vertex.
    procedure DrawLinked();
    begin
      for var i := 0 to Vertices.Count - 1 do Draw(Vertices.Item[i]);
    end;
    
    ///Draw vertex
    procedure DrawSelf() := Draw(self);
    
    ///Draw all lines those connect the current vertex to the others
    procedure DrawLines();
    begin
      Draw(self);
      for var i := 0 to Vertices.Count - 1 do
        if Vertices.Item[i] <> nil then
          Line(Position.Xposition, Position.Yposition, Vertices.Item[i].Position.Xposition, Vertices.Item[i].Position.Yposition);
    end;
    
    ///Add new vertex
    procedure AddVertex(v: GraphVertex):=Vertices.Add(v);
    
    ///Link two vertices
    procedure LinkTwoVertices(v: GraphVertex);
    begin
      AddVertex(v);
      v.AddVertex(self);
    end;
    
    ///Add new vertex in any array
    class procedure AddVertex(var a: array of GraphVertex; v: GraphVertex);
    begin
      SetLength(a, Length(a) + 1);
      a[Length(a) - 1] := v;
    end;
    
    ///Replace the first vertex with the second one
    procedure ResetVertex(v, v2: GraphVertex);
    var
      l, i: integer;
    begin
      l := Vertices.Count;
      i := 0;
      while (i < l) and (Vertices.Item[i] <> v) do Inc(i);
      Vertices.Item[i] := v2;
    end;
    
    ///Replace the first vertex with the second one in any array
    procedure ResetVertex(var a: array of GraphVertex; v, v2: GraphVertex);
    var
      l, i: integer;
    begin
      l := Length(a);
      i := 0;
      while (i < l) and (a[i] <> v) do Inc(i);
      a[i] := v2;
    end;
    
    ///Get vertex by it's id (name).
    class function GetVertex(a: array of GraphVertex; id: string): GraphVertex;
    var
      l, i: integer;
    begin
      l := Length(a);
      i := 0;
      while (i < l) and (a[i].Name <> id) do Inc(i);
      if i < l then GetVertex := a[i] else GetVertex := nil;
    end;
    
    ///Get vertex by it's level (vL).
    class function GetVertex(a: array of GraphVertex; vL: integer): GraphVertex;
    var
      l, i: integer;
    begin
      l := Length(a);
      i := 0;
      while (i < l) and (a[i].Level <> vL) do Inc(i);
      if i < l then GetVertex := a[i] else GetVertex := nil;
    end;
    
    ///Get all marked (not marked) vertexes from Vertexes.
    function GetMarkedVertexes(t: boolean): array of GraphVertex;
    var
      a: array of GraphVertex;
    begin
      for var i := 0 to Vertices.Count - 1 do
        if Vertices.Item[i].IsMarked = t then AddVertex(a, Vertices.Item[i]);
      GetMarkedVertexes := a;
    end;
  end;
end.

uses crt, BaseInterpolations, GraphOptimized, GraphABC;
const
  n = 8;
 
var
  a, b: array of GraphOptimized.GraphVertex;
  q: Stack<GraphOptimized.GraphVertex>;
  CurrVert: GraphOptimized.GraphVertex;
  LinkedVerts: array of GraphOptimized.GraphVertex;
 
///Получить вершину по имени.
function Get(name: string):= GraphOptimized.GraphVertex.GetVertex(a, name);
 
begin
  SetLength(a, n);
  for var i := 0 to n - 1 do begin a[i] := new GraphVertex(0, 0);a[i].ColorValue := RGB(255, 0, 0); end;
  
  a[0].Name := 'A';a[1].Name := 'B';a[2].Name := 'C';
  a[3].Name := 'D';a[4].Name := 'E';a[5].Name := 'F';
  a[6].Name := 'H';a[7].Name := 'K';
  
  Get('A').Position.Xposition := 200;Get('A').Position.Yposition := 100;
  Get('B').Position.Xposition := 400;Get('B').Position.Yposition := 100;
  Get('C').Position.Xposition := 500;Get('C').Position.Yposition := 400;
  Get('D').Position.Xposition := 600;Get('D').Position.Yposition := 300;
  Get('F').Position.Xposition := 700;Get('F').Position.Yposition := 400;
  Get('E').Position.Xposition := 400;Get('E').Position.Yposition := 500;
  Get('H').Position.Xposition := 100;Get('H').Position.Yposition := 400;
  Get('K').Position.Xposition := 300;Get('K').Position.Yposition := 400;
  
  Get('A').LinkTwoVertices(Get('B')); Get('A').LinkTwoVertices(Get('K')); Get('A').LinkTwoVertices(Get('H')); //Ошибка в этой строке происходит.
  Get('H').LinkTwoVertices(Get('K'));
  Get('B').LinkTwoVertices(Get('C'));
  Get('C').LinkTwoVertices(Get('D'));
  Get('C').LinkTwoVertices(Get('E'));
  Get('D').LinkTwoVertices(Get('F'));
  
  q := new Stack<GraphVertex>();
  q.Push(Get('A'));
  
  while (q.Count <> 0) and (CurrVert <> nil) and (CurrVert.Name <> 'C') do
  begin
    CurrVert := q.Pop;
    CurrVert.IsMarked := true;
    LinkedVerts := CurrVert.GetMarkedVertexes(false);
    for var i := 0 to Length(LinkedVerts) - 1 do
    begin
      LinkedVerts[i].Level := CurrVert.Level + 1;
      q.Push(LinkedVerts[i]);
    end;
  end;
  
  for var i := 0 to n - 1 do
  begin
    a[i].DrawSelf;a[i].DrawLines;
  end;
end.

  public 
    ///List of linked vertices
    Vertices := new List<GraphVertex>;