Initialize wxMDIParentFrame::m_accelWindowMenu in default ctor.

If wxMDIParentFrame object was created without its Create() ever being called,
it crashed in its dtor trying to delete an uninitialized pointer, so do
initialize it to NULL in all ctors (via a helper Init() method, as usual).

Closes #11216.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61986 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/mdi.h

@@ -24,7 +24,7 @@ class WXDLLIMPEXP_FWD_CORE wxAcceleratorTable;
 class WXDLLIMPEXP_CORE wxMDIParentFrame : public wxMDIParentFrameBase
 {
 public:
-    wxMDIParentFrame() { }
+    wxMDIParentFrame() { Init(); }
     wxMDIParentFrame(wxWindow *parent,
                      wxWindowID id,
                      const wxString& title,
@@ -33,6 +33,8 @@ public:
                      long style = wxDEFAULT_FRAME_STYLE | wxVSCROLL | wxHSCROLL,
                      const wxString& name = wxFrameNameStr)
     {
+        Init();
+
         Create(parent, id, title, pos, size, style, name);
     }
 
@@ -121,6 +123,9 @@ protected:
     void UpdateClientSize();
 
 private:
+    // common part of all ctors
+    void Init();
+
 #if wxUSE_MENUS
     // "Window" menu commands event handlers
     void OnMDICommand(wxCommandEvent& event);