Initialize wxAuiNotebookXmlHandler::m_isInside in ctor

See #15694.

Closes #17222.

(this is a backport of 7439c0951b from master)

docs/changes.txt

@@ -604,6 +604,7 @@ All (GUI):
 - Fix bug in wxImage::ClearAlpha() for shared data (Markus Rollmann).
 - Accept wxALIGN_CENTRE_HORIZONTAL in wxStaticText XRC handler (David Hart).
 - Fix appearance after updating a wxGrid with hidden rows/columns (iwbnwif).
+- Make wxAuiNotebookXmlHandler actually work.
 
 wxGTK:
 

src/xrc/xh_auinotbk.cpp

@@ -22,7 +22,8 @@
 wxIMPLEMENT_DYNAMIC_CLASS(wxAuiNotebookXmlHandler, wxXmlResourceHandler);
 
 wxAuiNotebookXmlHandler::wxAuiNotebookXmlHandler()
-    : wxXmlResourceHandler()
+    : wxXmlResourceHandler(),
+      m_isInside(false)
 {
     XRC_ADD_STYLE(wxAUI_NB_DEFAULT_STYLE);
     XRC_ADD_STYLE(wxAUI_NB_TAB_SPLIT);