Unity-Technologies/UnityCsReference · error · ArgumentNullException

The root is null

Error message

The root is null

What it means

SetDepthValuesForItems walks the tree from a root to assign depth values to each item; a null root would immediately cause a NullReferenceException when pushing it onto the stack, so the method fails fast with a descriptive ArgumentNullException instead. The guard is a standard precondition: depth computation cannot proceed without a well-defined root node.

Source

Thrown at Editor/Mono/GUI/TreeView/TreeViewUtililty.cs:23

using System;
using System.Collections.Generic;
using System.Linq;
using Unity.Collections;

namespace UnityEditor.IMGUI.Controls
{
    internal static class TreeViewUtility<TIdentifier> where TIdentifier : unmanaged, IEquatable<TIdentifier>
    {
        internal static void SetParentAndChildrenForItems(IList<TreeViewItem<TIdentifier>> rows, TreeViewItem<TIdentifier> root)
        {
            SetChildParentReferences(rows, root);
        }

        // For setting depths values based on children state of the items
        internal static void SetDepthValuesForItems(TreeViewItem<TIdentifier> root)
        {
            if (root == null)
                throw new ArgumentNullException("root", "The root is null");

            Stack<TreeViewItem<TIdentifier>> stack = new Stack<TreeViewItem<TIdentifier>>();
            stack.Push(root);
            while (stack.Count > 0)
            {
                TreeViewItem<TIdentifier> current = stack.Pop();
                if (current.children != null)
                {
                    foreach (var child in current.children)
                    {
                        if (child != null)
                        {
                            child.depth = current.depth + 1;
                            stack.Push(child);
                        }
                    }
                }
            }

View on GitHub (pinned to 225b0fbdb5)

Solutions

1. Ensure BuildRoot always returns a non-null root TreeViewItem before the tree view refreshes.
2. Null-check the root before calling SetDepthValuesForItems and skip/log instead of crashing.
3. Recreate the root (e.g. a default invisible root) when the data source is reset.
4. In tests, always pass a constructed root; use a guard assert in debug builds.

Example fix

// before
TreeViewUtility<int>.SetDepthValuesForItems(root);

// after
if (root == null)
    return;
TreeViewUtility<int>.SetDepthValuesForItems(root);

if (root == null) throw new ArgumentNullException(nameof(root));
TreeViewUtility<int>.SetDepthValuesForItems(root);

static bool HasRoot(TreeViewItem<int> root) => root != null;

When it happens

Trigger: Calling TreeViewUtility<T>.SetDepthValuesForItems(null); building a TreeViewItem hierarchy where the root failed to be constructed (e.g. BuildRoot returned null); reloading a tree view after the data source was cleared and root was set to null before the depth pass.

Common situations: A custom TreeView<T> whose BuildRoot returns null under an error condition; editor domain reload that nulls cached tree state; test scaffolding that constructs items without wiring a root.

Related errors

• Invalid argument: {dropPosition}
• Invalid tree for finding descendants: Ensure a complete tree
• gameObjects array is null
• GameObject in gameObjects array is null
• gameObject is null

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/f1d56c6fc5379c9a. Report an issue: GitHub.