fix(macos): prevent use-after-free crash on quit in GLCanvas3D teardown

Plater is a wxWindow subclass holding a pImpl (unique_ptr<priv> p). On app
exit the member p is destroyed before the wxWindow base destructor runs
DestroyChildren(), so the child View3D/GLCanvas3D windows are torn down
*after* p has already been freed.

GLCanvas3D::~GLCanvas3D() -> reset_volumes() then reaches back into the dead
Plater via two callbacks, each dereferencing the freed p:
  - Selection::clear() -> plater()->canvas3D() -> p->get_current_canvas3D()
  - _set_warning_notification() -> plater()->get_notification_manager()
    -> p->notification_manager
Which one fires depends on scene state (selection vs. volumes present), so
quitting with a model loaded reliably crashes with EXC_BAD_ACCESS.

Guard both callbacks with the existing wxGetApp().is_closing() flag (set in
MainFrame's close handler and GUI_App::shutdown(), both before
DoDelayedCleanup destroys the frame). Both calls are UI-only side effects
that are no-ops during shutdown, so normal-use behavior is unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/slic3r/GUI/GLCanvas3D.cpp

@@ -10060,6 +10060,12 @@ void GLCanvas3D::_set_warning_notification_if_needed(EWarning warning)
 
 void GLCanvas3D::_set_warning_notification(EWarning warning, bool state)
 {
+    // On app quit the Plater pImpl (p) is freed before its child GLCanvas3D windows are
+    // destroyed; reset_volumes() -> _set_warning_notification() -> get_notification_manager()
+    // would then dereference the freed p (use-after-free crash on exit). No warning
+    // notifications are needed while the app is closing.
+    if (wxGetApp().is_closing())
+        return;
     using NotificationLevel = NotificationManager::NotificationLevel;
     enum ErrorType{
         PLATER_WARNING,

src/slic3r/GUI/Selection.cpp

@@ -760,6 +760,12 @@ void Selection::clear()
 #endif
 
     // #et_FIXME fake KillFocus from sidebar
+    // On app quit the Plater's child GLCanvas3D windows are destroyed by the wxWindow
+    // base destructor *after* Plater's pImpl (p) has already been freed. reset_volumes()
+    // -> Selection::clear() then reaches back into plater()->canvas3D(), a use-after-free
+    // that crashes on exit. This is a UI-only redraw hint, so skip it while closing.
+    if (wxGetApp().is_closing())
+        return;
     wxGetApp().plater()->canvas3D()->handle_sidebar_focus_event("", false);
 }