refactor: Add conditional chaining to session-manager call

ui/desktop/electron-app/src/index.js

@@ -200,7 +200,7 @@ app.on('before-quit', (event) => {
         case 0:
           sessionManager.stopAll();
           break;
-        case 1:
+        default:
           event.preventDefault();
           break;
       }

ui/desktop/electron-app/src/models/session-manager.js

@@ -31,7 +31,7 @@ class SessionManager {
    */
   stopById(session_id) {
     const session = this.#sessions.find((session) => session.id === session_id);
-    return session && session.stop();
+    return session?.stop?.();
   }
 
   stopAll() {