We got a Code from our teacher which should represent a working Linked List. But the Problem is, that i can not Add anything. When i try to add something with an Button i get this Pup op. I tried to change aktpos with Liste but it did not work either. I am always getting this Error meassage:

This is my current Code:

unit Unit1;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, Forms, Controls, Graphics, Dialogs, StdCtrls;

type

  { TForm1 }

  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    Button3: TButton;
    Button4: TButton;
    Edit1: TEdit;
    Label1: TLabel;
     procedure deleClick(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure nachClick(Sender: TObject);
    procedure neuesClick(Sender: TObject);
    procedure prevClick(Sender: TObject);

  private

  public

  end;
   TListe= Class
    Zahl: Integer;
    next,prev: TListe;
    procedure einfuegen();
    procedure loeschen();
    procedure naechstes();
    procedure vorgaenger();
  end;

var
  Form1: TForm1;
  Liste, aktPos: TListe;

implementation

{$R *.lfm}

{ TForm1 }

procedure TForm1.FormCreate(Sender: TObject);
begin
  Liste:= TListe.create;
  Liste.Zahl:=0;
  Liste.prev:=Liste;
  Liste.next:= Liste;
  Form1.Edit1.Text:=IntToSTr(Liste.Zahl);
  aktPos:=Liste;
end;

procedure TForm1.nachClick(Sender: TObject);
begin
  aktpos.naechstes();
end;

procedure TForm1.neuesClick(Sender: TObject);
begin
   aktpos.einfuegen();
end;

procedure TForm1.prevClick(Sender: TObject);
begin
   aktpos.vorgaenger();
end;

procedure TForm1.deleClick(Sender: TObject);
begin
   aktpos.loeschen();
end;

procedure TListe.einfuegen();
var new,hilf:TListe;
  eingabe:string;
begin
  hilf:=aktpos.next;
  new:=TListe.create;
  Form1.Edit1.Text:='';
  repeat
    eingabe := InputBox('NeuesElement', 'Zahleingeben', '');
  until eingabe <> '';
  new.Zahl:=StrToInt(eingabe);
  new.next:=hilf;
  new.prev:=aktPos;
  aktpos.next:=new;
  hilf.prev:=new;
  Form1.Edit1.Text:=IntToSTr(aktpos.Zahl);
end;

procedure TListe.loeschen();
var hilf:TListe;
begin
   hilf:=aktPos;
   aktPos:=aktpos.next;
   aktpos.prev:=hilf.prev;
   aktpos.prev.next:=aktpos;
   hilf.destroy;
   Form1.Edit1.Text:=IntToSTr(aktpos.Zahl);
end;

procedure TListe.naechstes();
begin
   aktPos:=aktpos.next;
   Form1.Edit1.Text:=IntToSTr(aktPos.Zahl);
end;

procedure TListe.vorgaenger();
begin
   aktPos:=aktpos.prev;
   Form1.Edit1.Text:=IntToSTr(aktPos.Zahl);
end;

end.

end.
            ```
I am not sure if the Problem is perhabs on the lines before, therefore i am posting the complete Code. I tried my best to fix the Code but could not make it because it is my first time programming recursive.