Plater's pImpl (unique_ptr<priv> p) is destroyed before the wxWindow base
destructor runs DestroyChildren(), so child GLCanvas3D windows are torn down
after p is gone. GLCanvas3D::~GLCanvas3D() -> reset_volumes() then dereferences
the freed p through two paths:
- Selection::clear() -> plater()->canvas3D() -> p->get_current_canvas3D()
- _set_warning_notification() -> plater()->get_notification_manager()
Guard both with the existing wxGetApp().is_closing() flag; both 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>
