Error message:Control " has no parent control.

I learn component writing from the online help on Delphi. When I try to write my first component. But I get Control " has no parent window when I try to use it.  Although it is not completed, it should at least display something, not a error message. The source is:
unit ButtonPro;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  ExtCtrls;

type
  TButtonPro = class(TCustomControl)
  private
    { Private declarations }
    FNormalPic, FActivePic, FPressingPic, FDisabledPic, FDownPic: TBitmap;
    FBtnDown, FStretch: Boolean;
    FTranspColor, FBkColor: TColor;
    procedure UpdatePic(NewPic: TBitmap);
    procedure SetNormalPic(Value: TBitmap);
    procedure SetActivePic(Value: TBitmap);
    procedure SetPressingPic(Value: TBitmap);
    procedure SetDisabledPic(Value: TBitmap);
    procedure SetDownPic(Value: TBitmap);
    procedure SetTranspColor(Value: TColor);
    procedure SetBkColor(Value: TColor);
  protected
    { Protected declarations }
    procedure CMMouseEnter(var Message: TMessage); message CM_MOUSEENTER;
    procedure CMMouseLeave(var Message: TMessage); message CM_MOUSELEAVE;
    procedure WMLButtonDown(var Message: TMessage); message WM_LBUTTONDOWN;
    procedure WMLButtonUp(var Message: TMessage); message WM_LBUTTONUP;
  public
    { Public declarations }
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;

  published
    { Published declarations }
    Property NormalPicture: TBitmap read FNormalPic write SetNormalPic;
    Property ActivePicture: TBitmap read FActivePic write SetActivePic;
    Property PressingPicture: TBitmap read FPressingPic write SetPressingPic;
    Property DisabledPicture: TBitmap read FDisabledPic write SetDisabledPic;
    Property DownPicture: TBitmap read FDownPic write SetDownPic;
    Property Down: Boolean read FBtnDown write FBtnDown;
    Property Stretch: Boolean read FStretch write FStretch;
    Property TransparentColor: TColor read FTranspColor write SetTranspColor default clBackground;
    Property BackgroundColor: TColor read FBkColor write SetBkColor default clBackground;
    Property Height default 50;
    Property Width default 50;
    Property Visible;
  end;

procedure Register;

implementation

procedure Register;
begin
  RegisterComponents('My Components', [TButtonPro]);
end;

procedure TButtonPro.CMMouseEnter(var Message: TMessage);
begin
  UpdatePic(FActivePic);
  inherited;
end;

procedure TButtonPro.CMMouseLeave(var Message: TMessage);
begin
  If Enabled = False then
    UpdatePic(FDisabledPic)
  else if FBtnDown = True then
         UpdatePic(FDownPic)
       else UpdatePic(FNormalPic);
  inherited;
end;

procedure TButtonPro.WMLButtonDown(var Message: TMessage);
begin
  inherited;
  UpdatePic(FDownPic);
end;

procedure TButtonPro.WMLButtonUp(var Message: TMessage);
begin
  inherited;
  If not Enabled then
    UpdatePic(FDisabledPic)
  else if FBtnDown then
         UpdatePic(FDownPic)
       else UpdatePic(FNormalPic);
end;

constructor TButtonPro.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  Height := 50;
  Width := 50;
  FNormalPic := TBitmap.Create;
  FActivePic := TBitmap.Create;
  FPressingPic := TBitmap.Create;
  FDisabledPic := TBitmap.Create;
  FDownPic := TBitmap.Create;
  FBtnDown := False;
  FTranspColor := clBackground;
  FBkColor := clBackground;
  UpdatePic(FNormalPic);
end;

destructor TButtonPro.Destroy;
begin
  FNormalPic.Free;
  FActivePic.Free;
  FPressingPic.Free;
  FDisabledPic.Free;
  FDownPic.Free;
  inherited Destroy;
end;

procedure TButtonPro.UpdatePic(NewPic: TBitmap);
var TmpRect: TRect;
begin
  TmpRect := Rect(0,0,NewPic.Width,NewPic.Height);
  Canvas.Brush.Color := FBkColor;
  If NewPic.Empty then Canvas.FillRect(BoundsRect)
  else If FStretch then Canvas.BrushCopy(BoundsRect,NewPic,TmpRect,FTranspColor)
         else Canvas.BrushCopy(TmpRect,NewPic,TmpRect,FTranspColor);
  Invalidate;
end;

procedure TButtonPro.SetBkColor(Value: TColor);
begin
  FBkColor := Value;
  UpdatePic(FNormalPic);
end;

procedure TButtonPro.SetTranspColor(Value: TColor);
begin
  FTranspColor := Value;
  UpdatePic(FNormalPic);
end;

procedure TButtonPro.SetNormalPic(Value: TBitmap);
begin
  FNormalPic.Assign(Value);
  UpdatePic(FNormalPic);
  If not Stretch then Height := FNormalPic.Height;
end;

procedure TButtonPro.SetActivePic(Value: TBitmap);
begin
  FActivePic.Assign(Value);
end;

procedure TButtonPro.SetPressingPic(Value: TBitmap);
begin
  FPressingPic.Assign(Value);
end;

procedure TButtonPro.SetDisabledPic(Value: TBitmap);
begin
  FDisabledPic.Assign(Value);
end;

procedure TButtonPro.SetDownPic(Value: TBitmap);
begin
  FDownPic.Assign(Value);
end;

end.

What's wrong with my code? Please tell me. It is very easy for any people who know component writing.