try this

src/application.rs

@@ -17,15 +17,19 @@ mod imp {
     impl ObjectImpl for Application {}
 
     impl ApplicationImpl for Application {
+        // called once by the primary instance
+        fn startup(&self) {
+            self.parent_startup();
+            ui::Window::new(self.obj().as_ref());
+        }
+
+        // executed by the primary instance
         fn activate(&self) {
             self.parent_activate();
-            match self.obj().active_window() {
+            self.obj()
-                None => {
+                .active_window()
-                    let window = ui::Window::new(self.obj().as_ref());
+                .expect("no window was active")
-                    window.present();
+                .present();
-                }
-                Some(win) => win.present(),
-            }
         }
     }