Fix stale add_component target handling

CHANGELOG.md

@@ -8,6 +8,7 @@ All notable public changes to Nexus Unity are documented here.
 - Relicensed Nexus Unity from `GPL-3.0-only` to the `MIT` license to remove copyleft friction for commercial Unity studios. All prior contributors consented to the relicense.
 
 ### Fixed
+- `add_component` now returns a clear `GameObject not found` error for stale instance IDs instead of throwing a raw Unity null reference.
 - `batch_execute` now caps batches at 50 requests and rejects nested batch execution.
 - `create_primitive` now validates parent, transform, and material inputs before creating the GameObject, and Vector3 inputs accept `[x, y, z]` arrays as well as `{x, y, z}` objects.
 - `get_game_object` now returns transform state and a compact component list so agents can verify basic write operations with the cheap read-back call.

Editor/MCPServerMethods.Component.cs

@@ -33,6 +33,7 @@ private static JToken AddComponent(JToken p)
         {
             if (p == null || p["instance_id"] == null || p["component_name"] == null) throw new Exception("instance_id and component_name required");
             var go = MCPServerMethods.IdToObject(MCPServerMethods.ExtractId(p)) as GameObject;
+            if (go == null) throw new Exception("GameObject not found");
             Type type = FindType(p["component_name"].ToString());
             if (type == null) throw new Exception($"Type '{p["component_name"]}' not found");
             return new JObject { ["status"] = "Success", ["message"] = $"Added {p["component_name"]} to {Undo.AddComponent(go, type).name}" };

Tests~/Editor/ConsolidatedManagersTests.cs

@@ -403,6 +403,18 @@ public void GetGameObject_ReturnsTransformAndComponentsForReadBackVerification()
             Assert.IsTrue(components.Any(c => c["type"]?.ToString() == "BoxCollider"));
         }
 
+        [Test]
+        public void AddComponent_StaleInstanceId_ReturnsGameObjectNotFound() {
+            var go = CreateTestGameObject();
+            int staleId = go.GetInstanceID();
+            GameObject.DestroyImmediate(go);
+
+            var res = CallRaw("add_component", new JObject { ["instance_id"] = staleId, ["component_name"] = "BoxCollider" });
+
+            Assert.IsNotNull(res["error"], res.ToString());
+            StringAssert.Contains("GameObject not found", res["error"]["message"].ToString());
+        }
+
         [Test]
         public void UnityComponentManager_Inspect_ReturnsSuccess() {
             var go = CreateTestGameObject();