win32 Dll ....为什么我在MFC中调win32 Dll 中的一个窗体 关闭后 客户窗体不能响应了

Question

MFC ...
void CtestdllnewDlg::OnBnClickedButton2()
{
HINSTANCE dllInstance=NULL;
dllInstance=LoadLibrary(L"DLinkLibrary.dll");
typedef void (*MyDlg)(void *);
MyDlg mydlg=(MyDlg)GetProcAddress(dllInstance,"ShowDlg");
(*mydlg)((void *)this->m_hWnd);

FreeLibrary(dllInstance);


}Dll ...
extern "C" void _declspec(dllexport)  ShowDlg(void *hwnd)
{
MyExportClass::hwndDLG = CreateDialog(MyExportClass::hinst,MAKEINTRESOURCE(IDD_DLGFOREXPORT),NULL,(DLGPROC)DlgProc);
//MyExportClass::hwndDLG = CreateDialog(MyExportClass::hinst,MAKEINTRESOURCE(IDD_DLGFOREXPORT),(HWND)hwnd,(DLGPROC)DlgProc);
ShowWindow(MyExportClass::hwndDLG, SW_SHOW);
}//回调BOOL CALLBACK DlgProc(HWND hDlg, UINT message,WPARAM wParam, LPARAM lParam)
{
switch(message)
{ 
case WM_INITDIALOG:
return TRUE;
case WM_COMMAND:
if(LOWORD(wParam)==IDOK)
//DialogBox(hinst,MAKEINTRESOURCE(IDD_ABOUTBOX),hDlg,(DLGPROC)AboutProc);
// MessageBox(hDlg,"34" , "12", MB_OKCANCEL);
return TRUE;    case WM_CLOSE:
DestroyWindow(hDlg);
MyExportClass::hwndDLG =NULL;
return TRUE;
}
return FALSE;
}关闭DLL中窗体后客户窗体就熄火！！求虾哥虾姐指点！！