Invalid pointer operation

library dll;
 
uses
  SysUtils,
  Classes;
 
function peresech(x11,x12,x21,x22,y11,y12,y21,y22:extended):string; stdcall;
var
  u1,u2:extended;
  s:string;
  begin
      u1:=((x22-x21)*(y11-y21)-(x11-x21)*(y22-y21))/((y22-y21)*(x12-x11)-(x22-x21)*(y12-y11));
      u2:=((x12-x11)*(y11-y21)-(y12-y11)*(x11-x21))/((y22-y21)*(x12-x11)-(x22-x21)*(y12-y11));
      if (y22-y21)*(x12-x11)-(x22-x21)*(y12-y11)=0 then
        if ((x22-x21)*(y11-y21)-(x11-x21)*(y22-y21)=0)or((x12-x11)*(y11-y21)-(y12-y11)*(x11-x21)=0) then
 
           s:='отрезки лежат на одной прямой'
 
 
            else
 
            s:='Îòðåçêè ëåæàò íà ïàðàëëåëüíûõ ïðÿìûõ'  
 
        else
 
 
    if (u1>=0)and(u1<=1)xor(u2>=0)and(u2<=1) then s:='Îòðåçêè íå ïåðåñåêàþòñÿ';
 
 
    if (u1>=0)and(u1<=1)and(u2>=0)and(u2<=1) then
        s:='Òî÷êà ïåðåñå÷åíèÿ: ('+floattostrf((x11+u1*(x12-x11)),ffFixed,8,1)+';'+floattostrf((y11+u1*(y12-y11)),ffFixed,8,1)+')';
    result:=s;
    end;
   
  exports
  peresech;
 
{$R *.res}
 
begin
end.

unit Unit1;
 
interface
 
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, ExtCtrls, Buttons;
 
type
  TForm1 = class(TForm)
    img1: TImage;
    edt1: TEdit;
    edt2: TEdit;
    edt3: TEdit;
    edt4: TEdit;
    edt5: TEdit;
    edt6: TEdit;
    edt7: TEdit;
    edt8: TEdit;
    lbl1: TLabel;
    lbl2: TLabel;
    lbl3: TLabel;
    lbl4: TLabel;
    lbl5: TLabel;
    lbl6: TLabel;
    lbl7: TLabel;
    lbl8: TLabel;
    btn1: TButton;
    btn2: TBitBtn;
    txt1: TStaticText;
    edt9: TEdit;
    procedure btn1Click(Sender: TObject);
    procedure edt1Change(Sender: TObject);
    procedure edt2KeyPress(Sender: TObject; var Key: Char);
 
  private
    { Private declarations }
  public
    { Public declarations }
  end;
 //function peresech(x11,x12,x21,x22,y11,y12,y21,y22:extended):string; stdcall;
 
var
  Form1: TForm1;
 
implementation
 function peresech(x11,x12,x21,x22,y11,y12,y21,y22:extended):string; stdcall; external'dll.dll'
{$R *.dfm}
 
 
procedure TForm1.btn1Click(Sender: TObject);
const
  mash=10;
var
  x11,x12,x21,x22,y11,y12,y21,y22,x0,y0:Integer;
  s:string;
 
begin
  //î÷èñòèòü ðèñóíîê
  img1.Canvas.Pen.Color:=clWhite;
  img1.Canvas.Rectangle(0,0,ClientWidth,ClientHeight);
  //ñ÷èòûâàåì êîîðäèíàòû
  x11:=StrToInt(edt2.Text);
  y11:=StrToInt(edt1.Text);
  x12:=StrToInt(edt3.Text);
  y12:=StrToInt(edt4.Text);
  x21:=StrToInt(edt5.Text);
  x22:=StrToInt(edt6.Text);
  y21:=StrToInt(edt7.Text);
  y22:=StrToInt(edt8.Text);
  //íàõîäèì öåíòð
  x0:=img1.Width div 2;
  y0:=img1.Height div 2;
  with img1.Canvas do
  begin
    Pen.Color:=clBlack;
    Pen.Width:=2;
    MoveTo(x0,0);
    LineTo(x0,ClientHeight);
    MoveTo(0,y0);
    LineTo(ClientWidth,y0);
    Pen.Color:=clPurple;
    Pen.width:=3;
    MoveTo((x11*mash)+x0,(-y11*mash)+y0);
    LineTo((x12*mash)+x0,(-y12*mash)+y0);
    Pen.Color:=clAqua;
    MoveTo((x21*mash)+x0,(-y21*mash)+y0);
    LineTo((x22*mash)+x0,(-y22*mash)+y0);
  end;
  s:=peresech(x11,x12,x21,x22,y11,y12,y21,y22);
  edt9.text:=s;
 
end;
 
 
procedure TForm1.edt1Change(Sender: TObject);
begin
 if (edt1.text='')or (edt2.text='') or (edt4.text='')
 or (edt5.text='') or (edt6.text='') or (edt7.text='')
 or(edt8.text='')
 then btn1.Enabled:=False
 else btn1.Enabled:=True;
end;
 
procedure TForm1.edt2KeyPress(Sender: TObject; var Key: Char);
begin
  case Key of
  '0'..'9',#8:;
  else
    key:=chr(0);
  end;
end;
end.