I can't run my simple application with the About form in the Main Form's About Menu

Question

I can't run my simple application with the About form in the About Menu. After creating the About Form with the Main unit also created, when I run it, only the About dialog box appears. I prepared the menus, made the associations in the onclick and also in the main menu help and created the About Form. But when it comes time to run it, only the About menu appears.

unit UnitPrincipal;

interface

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

type
    TFormPrincipal = class(TForm)
      Logotipo: TImage;
      Label1: TLabel;
      MainMenu1: TMainMenu;
      MenuSistema: TMenuItem;
      MenuSocios: TMenuItem;
      MenuMatricula: TMenuItem;
      MenuRelatorios: TMenuItem;
      MenuHelp: TMenuItem;
      MenuAtividades: TMenuItem;
      SistemaBackup: TMenuItem;
      SistemaRestaurar: TMenuItem;
      Separador: TMenuItem;
      SistemaFinalizar: TMenuItem;
      SociosAlteracao: TMenuItem;
      SociosExclusao: TMenuItem;
      SociosConsulta: TMenuItem;
      AtividadesCadastro: TMenuItem;
      AtividadesAlteracao: TMenuItem;
      AtividadesExclusao: TMenuItem;
      AtividadesConsulta: TMenuItem;
      MatriculaCadastro: TMenuItem;
      MatriculaExclusao: TMenuItem;
      MatriculaConsulta: TMenuItem;
      MatriculaSocios: TMenuItem;
      RelatoriosAtividades: TMenuItem;
      RelatoriosMatriculas: TMenuItem;
      HelpTopicos: TMenuItem;
      HelpSobre: TMenuItem;
      procedure SistemaFinalizarClick(Sender: TObject);
      procedure HelpSobreClick(Sender: TObject);
    private
      { Private declarations }
    public
      { Public declarations }
    end;

var
    FormPrincipal: TFormPrincipal;

implementation

  {$R *.dfm}

procedure TFormPrincipal.SistemaFinalizarClick(Sender: TObject);
begin
    Application.Terminate;
end;

procedure TFormPrincipal.HelpSobreClick(Sender: TObject);
begin
     FormSobre.ShowModal;
end;

end.

    unit UnitSobre;

    interface

    uses Windows, SysUtils, Classes, Graphics, Forms, Controls, 
    StdCtrls,
    Buttons, ExtCtrls;

    type
       TAboutBox = class(TForm)
       Panel1: TPanel;
       ProgramIcon: TImage;
       ProductName: TLabel;
       Version: TLabel;
       Copyright: TLabel;
       Comments: TLabel;
       OKButton: TButton;
       private
            { Private declarations }
       public
            { Public declarations }
       end;

    var
       AboutBox: TAboutBox;

    implementation

    {$R *.dfm}

    end.