关于连续两次打开dll中子窗口的mainmenu菜单后报错的问题

我在主程序exe中打开dll中子窗口的mainmenu菜单后关闭这个子窗口，然后重复这个操作就报错，也就是不能连续调用两次，但只要不用子窗口的mainmenu或者主程序exe中调用窗不含有mainmenu就不会报错，那位大哥帮我瞧一下呀//dll的调用程序如下
unit MainUnit;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, ExtCtrls, Menus, publicUnit, Grids, DB, ADODB,
  DBClient, MConnect, SConnect;type
  TfrmMain = class(TForm)
    MainMenu1: TMainMenu;
    N1: TMenuItem;
    Panel1: TPanel;
    Button1: TButton;
    Button2: TButton;
    procedure N1Click(Sender: TObject);
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
   private
     procedure WMGetDlgCode(var msgIn: TWMGetDlgCode); message WM_GETDLGCODE;
  public
    { Public declarations }
 end;
type
tnet=array[0..3] of pchar;
var
  frmMain: TfrmMain;
function ShowDllForm(App: TApplication; ACaption: string; sc: TScreen): Longint;
stdcall; external 'prjDll.dll';
//function openform(App:TApplication;sc:tscreen;netpara:tnet):Longint;
//stdcall;External'..\rzbinfo\reportbuild\reportbuild.dll';
//function openForm(App: TApplication;sc: TScreen;netpara:tnet): Longint;
//stdcall;external'..\rzbinfo\reportbuild\reportbuild.dll';
implementation{$R *.dfm}procedure TfrmMain.N1Click(Sender: TObject);
begin
  ShowDllForm(Application, '子窗体', Screen);
end;
procedure TfrmMain.WMGetDlgCode(var msgIn: TWMGetDlgCode);
begin
  inherited;
  msgIn.Result := msgIn.Result or DLGC_WANTTAB;
end;procedure TfrmMain.Button1Click(Sender: TObject);
//var
//netp:tnet;
begin
{netp[0]:=pchar(socketconnection1.Address);
netp[1]:=pchar(inttostr(socketconnection1.Port));
netp[2]:=pchar(socketconnection1.ServerName);
netp[3]:=pchar(socketconnection1.ServerGUID);
openForm(Application,Screen,netp);}
end;procedure TfrmMain.Button2Click(Sender: TObject);
begin
  ShowDllForm(Application, '子窗体', Screen);
end;end.//dll如下
library prjDLL;
uses
  SysUtils,
  Classes,
  Windows,
  Forms,
  childUnit in 'childUnit.pas' {frmChild},
  Unit1 in 'Unit1.pas' {Form1};{$R *.res}var
  DLLApp: TApplication;
  DLLScreen:TScreen;function ShowDllForm(App:TApplication; ACaption:String; sc:TScreen):Longint;stdcall;
begin
  Application := App;
  Screen:=sc;  if not Assigned(frmChild) then
    frmChild:=TfrmChild.Create(Application);
  Result:=Longint(frmChild);
  frmChild.Caption := ACaption;
  frmChild.Show;
end;procedure DLLUnloadProc(Reason : Integer);
begin
  if Reason = DLL_PROCESS_DETACH then begin
    Application := DLLApp;//恢复
    Screen:=DLLScreen;
  end;
end;
exports
  ShowDllForm;begin
  DLLApp := Application; //保存 DLL 中初始的 Application 对象
  DLLScreen:=Screen;
  DLLProc := @DLLUnloadProc;     //保证 DLL 卸载时恢复原来的 Application
end.
//frmchild如下unit childUnit;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, publicUnit, Grids, DB, ADODB, DBGrids, Menus, StdCtrls;type
  TfrmChild = class(TForm)
    ComboBox1: TComboBox;
    MainMenu1: TMainMenu;
    xxx1: TMenuItem;
    xxxx1: TMenuItem;
    xxxx2: TMenuItem;
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
    procedure FormKeyPress(Sender: TObject; var Key: Char);
    procedure FormCreate(Sender: TObject);
    procedure xxxx2Click(Sender: TObject);
  private
    procedure WMGetDlgCode(var msgIn: TWMGetDlgCode);
      message WM_GETDLGCODE;
  public  end;var
  frmChild: TfrmChild;implementationuses Unit1;{$R *.dfm}procedure TfrmChild.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  FreeAndNil(frmChild);
end;procedure TfrmChild.FormKeyPress(Sender: TObject; var Key: Char);
begin
  if Key = #9 then // Char(VK_tab) then
    if not (ActiveControl is TDBGrid) then
    begin
      Key := #0;
      Perform(WM_NEXTDLGCTL, 0, 0);
    end;
end;procedure TfrmChild.WMGetDlgCode(var msgIn: TWMGetDlgCode);
begin
  inherited;
  msgIn.Result := msgIn.Result or DLGC_WANTTAB;
end;procedure TfrmChild.FormCreate(Sender: TObject);
begin
combobox1.Items.AddStrings(screen.fonts);
  Self.KeyPreview := True;
end;procedure TfrmChild.xxxx2Click(Sender: TObject);
begin
form1:=tform1.create(self);
form1.showmodal;
form1.free;
end;end.
//form1就是一个空白的tform;