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procedure TForm1.FormCreate(Sender: TObject);
var f: textfile;
begin
  if fileexists('d:\Highscore.txt') then
    highscore.ListBox1.Items.LoadFromFile('d:\Highscore.txt') //<= an dieser Stelle tritt                  die exception auf!
  else begin
    assignfile(f, 'd:\Highscore.txt');
    rewrite(f);
    closefile(f);
  end;
end;					

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procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  highscore.ListBox1.Items.SaveToFile('d:\Highscore.txt');
end;					

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 {...}
begin
 highscore.Create(form1);  // oder highscore.create(highscore)? - es klappt beides nicht!
 if fileexists('d:\Highscore.txt') then begin
 highscore.ListBox1.Items.LoadFromFile('d:\Highscore.txt')
 {...}					

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uses highscore;
{...}
begin
  Application.CreateForm(Thighscore, highscore); // hier gegebenenfalls Thighscore auf TForm2 anpassen
  if fileexists('d:\Highscore.txt') then begin
  highscore.ListBox1.Items.LoadFromFile('d:\Highscore.txt')
{...}					

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uses highscore;
{...}
var
  highscore: Thighscore; // ggf Thighscore auf TForm2 ändern
{...}
begin
  highscore := Thighscore.Create(form1);  // als Parent kannste auch Application.MainForm, nil, etc angeben. ggf. Thighscore auf TForm2 anpassen
  if fileexists('d:\Highscore.txt') then begin
  highscore.ListBox1.Items.LoadFromFile('d:\Highscore.txt')
{...}					

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highscore.Free;