Ошибка: E2003 undeclared identifier

unit Unit1;
 
interface
 
uses
  Windows, Messages, SysUtils, Variants, Classes, Controls, Forms,
  Dialogs, StdCtrls, ExtCtrls;
 
type
  TForm1 = class(TForm)
    btn5: TButton;
    Button1: TButton;
    Button2: TButton;
    Button3: TButton;
    Button4: TButton;
 
    procedure btn5Click(Sender: TObject);
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
    procedure Button4Click(Sender: TObject);
 
  private
    fRestore: Boolean;
  public
  end;
 
 
 
   //Здесь необходимо описать класс TMyThread:
  TMyThread = class(TThread)
    private
    { Private declarations }
  protected
    procedure Execute; override;
  end;
 
 
 
var
  Form1: TForm1;
  MyThread: TMyThread;
  _PluginProc: function(Code: Cardinal; p1, p2, p3: widestring): widestring; stdcall;
implementation
 
{$R *.dfm}
 
// создание приостановленного потока
constructor TMyThread.Create(CreateSuspended: boolean);
begin
  inherited Create(CreateSuspended);
  FreeOnTerminate:= true;
end;
  // приостановить поток
procedure TForm1.Button2Click(Sender: TObject);
begin
 Thread.Suspend;
end;
 //  возобновить поток
procedure TForm1.Button3Click(Sender: TObject);
begin
Thread.Resume;
end;
 // остановить поток
procedure TForm1.Button4Click(Sender: TObject);
begin
Thread.Terminate;
end;
// запуск потока
procedure TForm1.Button1Click(Sender: TObject);
begin
with TMyThread.Create(True) do
        FreeOnTerminate:=true;
end;
 
procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  Action := caNone;
  Hide;
end;
 
 
function PluginProc(Code: Cardinal; p1: widestring = ''; p2: widestring = ''; p3: widestring = ''): widestring;
begin
  Result := _PluginProc(Code, p1, p2, p3);
end;
 
 // поток
    procedure TMyThread.Execute();
   begin
   while 1<>2 do begin  if not Terminated then
   begin
    PluginProc(0);
    sleep(6000) ;
    end;
   end;
   end;
 
 
 
  // закрытие окна
procedure TForm1.btn5Click(Sender: TObject);
begin
  Close;
end;
 
 
end.