Fix crash when reparenting the focused window to another TLW

If the window stored as m_winLastFocused in one TLW was reparented to
another one and then destroyed, this pointer to it wasn't updated and
became dangling.

Fix this by using a safe weak reference instead of raw pointer for
m_winLastFocused. This ensures that it can never be used when it becomes
invalid.

Closes #17980.

docs/changes.txt

@@ -214,6 +214,7 @@ wxMSW:
 - Fix updating radio groups when non-radio item is inserted to wxMenu.
 - Fix autoselecting the contents of wxTextCtrl with wxWANTS_CHARS style.
 - Implement SetIcon(), SetPosition(), GetPosition() for native wxProgressDialog.
+- Fix crash when reparenting the currently focused window to another TLW.
 
 wxOSX:
 

include/wx/msw/toplevel.h

@@ -11,6 +11,8 @@
 #ifndef _WX_MSW_TOPLEVEL_H_
 #define _WX_MSW_TOPLEVEL_H_
 
+#include "wx/weakref.h"
+
 // ----------------------------------------------------------------------------
 // wxTopLevelWindowMSW
 // ----------------------------------------------------------------------------
@@ -176,7 +178,7 @@ protected:
     // The last focused child: we remember it when we're deactivated and
     // restore focus to it when we're activated (this is done here) or restored
     // from iconic state (done by wxFrame).
-    wxWindow             *m_winLastFocused;
+    wxWindowRef m_winLastFocused;
 
 private:
 

src/msw/toplevel.cpp

@@ -102,8 +102,6 @@ void wxTopLevelWindowMSW::Init()
     m_fsIsMaximized = false;
     m_fsIsShowing = false;
 
-    m_winLastFocused = NULL;
-
     m_menuSystem = NULL;
 }
 
@@ -1195,7 +1193,9 @@ void wxTopLevelWindowMSW::DoRestoreLastFocus()
         parent = this;
     }
 
-    wxSetFocusToChild(parent, &m_winLastFocused);
+    wxWindow* winPtr = m_winLastFocused;
+    wxSetFocusToChild(parent, &winPtr);
+    m_winLastFocused = winPtr;
 }
 
 void wxTopLevelWindowMSW::OnActivate(wxActivateEvent& event)