GetDlgItem() get NULL the 3rd time after first 2 successes

Question

vs2008,win7 32bits,vc++

I use a CPropertyPage dlg on a CPropertySheet;

The dlg have a ActiveX WebBrowser control and a button1;

 ON_BN_CLICKED(IDC_BTN1,OnBnClickedBtnDisp)

void DlgParaAuditPage_Layout::OnBnClickedBtnDispstd()

{

 m_bDisplay=!m_bDisplay;
 CWnd*pWeb=GetDlgItem(IDC_EXPLORER); //ActiveX WebBrowser control
 ASSERT(pWeb);
 pWeb->ShowWindow(m_bDisplay?SW_SHOW:SW_HIDE);

}

the original state  m_bDisplay==false;

when I press IDC_BTN1 ->1 time, pWeb show

->2 time, pWeb Hide

But when I ->3rd time press,then ASSERT(pWeb) fired!

I just don't know why?

Any suggestion is good!

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Jzq