[sim] handle missing object in sim_force_remove (same race as sim_poke)

Author: zeeshanlakhani

sled-agent/src/sim/collection.rs

@@ -221,7 +221,13 @@ impl<S: Simulatable + 'static> SimCollection<S> {
     /// any further state changes for it.
     pub async fn sim_force_remove(&self, id: Uuid) {
         let mut objects = self.objects.lock().await;
-        let object = objects.remove(&id).unwrap();
+        // The object may already have been destroyed and removed by a
+        // concurrent `sim_poke` (e.g., a background `sim_step` racing with
+        // an explicit `instance_unregister`). In that case there is
+        // nothing left to do.
+        let Some(object) = objects.remove(&id) else {
+            return;
+        };
         if let Some(mut tx) = object.channel_tx {
             tx.close_channel();
         }