Stack overflow on delphi button click

I am creating an application that shrinks the pixel width.
When I click this app button two or three times, a message pops up and says Stack Overflow.

Here's the message:

Error line in my application

Here's my code:

procedure TForm1.cariThin();
var
  baris_gbr, kolom_gbr, x, y, a, b, i, j, p1, p2, n : integer;
  imgval : array [0..500,0..500] of integer;
  mark : array [0..500,0..500] of integer;
  nb : array [1..9] of integer;
  hasdelete: boolean;
  R, G, BL, AB : integer;
begin
  Image3.Width := Image1.Width;
  Image3.Height := Image1.Height;

  baris_gbr := Image1.Picture.Height;
  kolom_gbr := Image1.Picture.Width;


  For kolom_gbr:= 0 To image1.Width - 1 Do
  Begin
    For baris_gbr:= 0 To image1.Height - 1 Do
     Begin
      R:= GetRValue(image1.Canvas.Pixels[kolom_gbr, baris_gbr]);
      G:= GetGValue(image1.Canvas.Pixels[kolom_gbr, baris_gbr]);
      BL:= GetBValue(image1.Canvas.Pixels[kolom_gbr, baris_gbr]);
      AB:= (R + G + BL) Div 3;

      if (AB > 200) then
      begin
        Image1.Canvas.Pixels[kolom_gbr, baris_gbr] := rgb(255,255,255);
      end
      else
      begin
        Image1.Canvas.Pixels[kolom_gbr, baris_gbr] := rgb(0,0,0);
      end;
    End;
  End;

  for y := 0 to baris_gbr-1 do
  begin
    for x := 0 to kolom_gbr-1 do
    begin
      if (Image1.canvas.pixels[x,y] = clBlack) then
      begin
        imgval[x,y] := 1;
      end
      else
      begin
         imgval[x,y] := 0;
      end;
    end;
  end;

  hasdelete := True;
  while (hasdelete) do
  begin
    hasdelete := False;
    for y := 0 to baris_gbr-1 do
    begin
     for x := 0 to kolom_gbr-1 do
     begin
        if (imgval[x,y] = 1) then
        begin
          for n:=1 to 8 do
          begin
            nb[n] := 0;
            nb[1] := imgval[x,y];
            nb[2] := imgval[x,y-1];
            nb[3] := imgval[x+1,y-1];
            nb[4] := imgval[x+1,y];
            nb[5] := imgval[x+1,y+1];
            nb[6] := imgval[x,y+1];
            nb[7] := imgval[x-1,y+1];
            nb[8] := imgval[x-1,y];
            nb[9] := imgval[x-1,y-1];
            a := 0;
          end;

          for i:= 2 to 8 do
          begin
            if ((nb[i] = 0) AND (nb[i+1] = 1)) then
            begin
              inc(a);
            end;
          end;

          if ((nb[9] = 0) AND (nb[2] = 1)) then
          begin
            inc(a);
          end;

          b := nb[2] + nb[3] + nb[4] + nb[5] + nb[6] + nb[7] + nb[8] + nb[9];
          p1 := nb[2] * nb[4] * nb[6];
          p2 := nb[4] * nb[6] * nb[8];

          if ((a = 1) AND ((b>=2) AND (b <= 6)) AND (p1 = 0) AND (p2 = 0)) then
          begin
            mark[x,y] := 0;
            hasdelete := true;
          end
          else
          begin
            mark[x,y] := 1;
          end
        end
        else
        begin
          mark[x,y] := 0;
        end;
      end;
    end;

    for y:=0 to baris_gbr-1 do
    begin
      for x:=0 to kolom_gbr-1 do
      begin
        imgval[x,y] := mark[x,y];
      end;
    end;
  end;
end;

      

        
        
        
      

    

Why does my application keep talking about overflow? is there any solution to fix it? or can we have an exception handler? thank

EDIT
My expression is now talking about an access violation.

He raised an error on this line: nb [7]: = imgval [x-1, y + 1]; why did this happen?