Unity-Technologies/UnityCsReference · error · ArgumentNullException

Prefab source must not be null.

Error message

Prefab source must not be null.

What it means

Thrown by ApplyRemovedGameObject(GameObject gameObjectInInstance, GameObject assetGameObject, InteractionMode) when assetGameObject is null — ArgumentNullException(nameof(assetGameObject)). The asset GameObject identifies which child was removed in the instance; without it Unity cannot map the removal back to the source hierarchy.

Source

Thrown at Editor/Mono/Prefabs/PrefabUtility.cs:1566

        }

        private static void RemoveRemovedGameObjectOverridesWhichAreNull(Object prefabInstanceObject)
        {
            var removedGameObjects = PrefabUtility.GetRemovedGameObjects(prefabInstanceObject);
#pragma warning disable UA2001 // The Banned API Analyzer produces compile errors for any new Linq code. This pre-existing usage has been suppressed, but should be rewritten if possible.
            var filteredRemovedGameObjects = (from go in removedGameObjects where go != null select go).ToArray();
#pragma warning restore UA2001
            PrefabUtility.SetRemovedGameObjects(prefabInstanceObject, filteredRemovedGameObjects);
        }

        public static void ApplyRemovedGameObject(GameObject gameObjectInInstance, GameObject assetGameObject, InteractionMode action)
        {
            DateTime startTime = DateTime.UtcNow;

            ThrowExceptionIfNotValidPrefabInstanceObject(gameObjectInInstance, true);

            if (assetGameObject == null)
                throw new ArgumentNullException(nameof(assetGameObject), "Prefab source must not be null.");
            if (!IsPrefabInstanceObjectOf(gameObjectInInstance, PrefabUtility.GetPrefabAssetHandle(assetGameObject)))
                throw new ArgumentException("Prefab instance must match Prefab source.");
            if (assetGameObject.transform.root == assetGameObject.transform)
                throw new ArgumentException("The asset GameObject cannot be the root as the root cannot be removed as an override.");

            var actionName = "Apply Prefab removed GameObject";
            var prefabInstanceObject = PrefabUtility.GetPrefabInstanceHandle(gameObjectInInstance);
            GameObject prefabInstanceRoot = GetOutermostPrefabInstanceRoot(gameObjectInInstance);

            if (prefabInstanceObject == null)
                throw new ArgumentNullException(nameof(prefabInstanceObject), "Prefab instance must not be null.");

            PrefabUtility.Internal_CallPrefabInstanceApplying(prefabInstanceRoot);

            string assetPath = AssetDatabase.GetAssetPath(assetGameObject);
            GameObject assetRoot = GetRootGameObject(assetGameObject);
            byte[] originalFileContent = null;

View on GitHub (pinned to 225b0fbdb5)

Solutions

1. Null-check assetGameObject before calling: if (assetGameObject == null) return;
2. Resolve assetGameObject from a freshly loaded prefab asset before the call.
3. Confirm gameObjectInInstance is itself a valid prefab instance object (ThrowExceptionIfNotValidPrefabInstanceObject runs first).

Example fix

// before
PrefabUtility.ApplyRemovedGameObject(instanceGo, assetGo, InteractionMode.UserAction);

// after
if (assetGo == null) return;
PrefabUtility.ApplyRemovedGameObject(instanceGo, assetGo, InteractionMode.UserAction);

if (assetGameObject == null) { Debug.LogWarning("Prefab source GameObject is null."); return; }
PrefabUtility.ApplyRemovedGameObject(gameObjectInInstance, assetGameObject, action);

static bool IsLiveAssetGameObject(GameObject assetGo)
    => assetGo != null && EditorUtility.IsPersistent(assetGo);

When it happens

Trigger: Calling ApplyRemovedGameObject with a null assetGameObject; passing a destroyed GameObject reference; forwarding a Find/GetComponent lookup that returned null.

Common situations: Tooling applying 'removed GameObject' overrides where the asset-side lookup failed; stale references after the asset hierarchy changed; passing the instance GameObject where the asset GameObject was expected.

Related errors

• instanceRoots
• prefabInstances
• Cannot apply added component. Component is null.
• Cannot revert added component. Component is null.
• Prefab source may not be null.

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/5aed0f32e604f5c1. Report an issue: GitHub.