fixed crash in wxHtmlHelpController if the help window is still open

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@52470 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/html/helpwnd.cpp

@@ -592,6 +592,9 @@ bool wxHtmlHelpWindow::Create(wxWindow* parent, wxWindowID id,
 
 wxHtmlHelpWindow::~wxHtmlHelpWindow()
 {
+    if ( m_helpController )
+        m_helpController->SetHelpWindow(NULL);
+
     delete m_mergedIndex;
 
     // PopEventHandler(); // wxhtmlhelpcontroller (not any more!)