DLL调用正常，但在freelibrary出错。

在程序中我动态调用DLL中的函数，DLL文件中包含一个form,引出的函数将建立一个form,执行都很正常，但在freelibrary时出错，提示如下：
Project e:\delphi\test.exe raised too many consecutive exceptions:'access violation at 0x00000000:read of address 0x00000000',Process stopped.
后来经过各种方法试验，发现不用freelibrary这条语句，程序就执行正常。我觉得奇怪，就试着运行delphi5开发人员指南中的DLL一章的例子，也出现同样的错误。Delphi5开人员指南的例子如下：
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DLL代码
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library CalendarLib;

uses
ShareMem,
SysUtils,
Classes,
DLLFrm in 'DLLFrm.pas' {DLLForm};

exports
ShowCalendar;

begin
end.

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DLLFrm.pas代码
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unit DLLFrm;

interface

uses
SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls,
Forms, Dialogs, Grids, Calendar;

type

TDLLForm = class(TForm)
calDllCalendar: TCalendar;
procedure calDllCalendarDblClick(Sender: TObject);
end;

{ Declare the export function }
function ShowCalendar(AHandle: THandle; ACaption: String): TDateTime; StdCall;

implementation
{$R *.DFM}

function ShowCalendar(AHandle: THandle; ACaption: String): TDateTime;
var
DLLForm: TDllForm;
begin
// Copy application handle to DLL's TApplication object
Application.Handle := AHandle;
DLLForm := TDLLForm.Create(Application);
try
DLLForm.Caption := ACaption;
DLLForm.ShowModal;
Result := DLLForm.calDLLCalendar.CalendarDate; // Pass the date back in Result
finally
DLLForm.Free;
end;
end;

procedure TDLLForm.calDllCalendarDblClick(Sender: TObject);
begin
Close;
end;

end.

=================================
调用程序代码
=================================
unit MainFfm;

interface

uses
SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls,
Forms, Dialogs, StdCtrls;

type
TShowCalendar = function (AHandle: THandle; ACaption: String): TDateTime; StdCall;

EDLLLoadError = class(Exception);

TMainForm = class(TForm)
lblDate: TLabel;
btnGetCalendar: TButton;
procedure btnGetCalendarClick(Sender: TObject);
end;

var
MainForm: TMainForm;

implementation

{$R *.DFM}

procedure TMainForm.btnGetCalendarClick(Sender: TObject);
var
LibHandle : THandle;
ShowCalendar: TShowCalendar;
begin
LibHandle := LoadLibrary('CALENDARLIB.DLL');
try
if LibHandle = 0 then
raise EDLLLoadError.Create('Unable to Load DLL');
@ShowCalendar := GetProcAddress(LibHandle, 'ShowCalendar');
if not (@ShowCalendar = nil) then
lblDate.Caption := DateToStr(ShowCalendar(Application.Handle, Caption))
else
RaiseLastWin32Error;
finally
FreeLibrary(LibHandle); //这一句出错
end;
end;

开始我以为是参数类型为String的问题，但后来试着将String改为Pchar，还是出现同样的问题。
实在是不知该问题出在哪里，还请大侠们指点，谢谢！