Help needed on GetDocument

[0xC0000005]

//
// In your view (or document, or frame, etc.)
//
CMyView::OnDialogBox()
{
  CMyDialog dlg(GetDocument());
  dlg.DoModal();
}

//
// In your dialog box header file:
//
class CMyDocument;

class CDialog : public CDialog
{
  // Modify the normal constructor
  CMyDialog(CMyDocument *pDoc, CWnd *pParent=NULL);
  // Don't allow default constructor
  CMyDialog(){ ASSERT(FALSE); }

protected:
  CMyDocument *m_pDoc;
}

//
// In your dialog box cpp file
//
CMyDialog::CMyDialog(CMyDocument *pDoc, CWnd *pParent/*=NULL*/)
  :: CDialog(CMyDialog::IDD,pParent), m_pDoc(pDoc)
{
}