problem.rs

// First some setup code

  pub struct PwvucontrolApplication {
      pub(crate) window: OnceCell<PwvucontrolWindow>,
    }
  
   impl ApplicationImpl for PwvucontrolApplication {
        fn activate(&self) {
            let window = self
            .window
            .get()
            .expect("Should always be initialized in gio_application_startup");

            // Ask the window manager/compositor to present the window
            window.present();
            self.setup_wp_connection();
        }

        fn startup(&self) {
            self.parent_startup();
    
            let window = PwvucontrolWindow::new(&self.obj());
            self.window
                .set(window)
                .expect("Failed to initialize application window");
        }
    }
        
        
// Now the part that breaks
        
    impl PwvucontrolApplication {
        fn setup_wp_connection(&self) {
            wp::core::Core::init();

            wireplumber::Log::set_default_level("3");

            let props = wp::pw::Properties::new_string("media.category=Manager");

            let wp_core = wp::core::Core::new(Some(&glib::MainContext::default()), Some(props));
            let wp_om = wp::registry::ObjectManager::new();

            wp_core.connect_local("connected", false, |_obj| {
                let app = PwvucontrolApplication::default();
                let win = app.window.get().expect("window"); // PANICS HERE!
                win.imp().viewstack.set_visible_child_name("connected");
                None
            });
        }
        
// For reference the default impl:
        
  impl Default for PwvucontrolApplication {
    fn default() -> Self {
        gio::Application::default()
            .expect("Could not get default GApplication")
            .downcast()
            .unwrap()
    }
}