AvaloniaUI/Avalonia · error · ArgumentException

Cannot add null to LogicalChildren.

Error message

Cannot add null to LogicalChildren.

What it means

Error "Cannot add null to LogicalChildren." thrown in AvaloniaUI/Avalonia.

Source

Thrown at src/Avalonia.Base/StyledElement.cs:758

        {
            while (e != null)
            {
                if (e is ILogicalRoot root)
                {
                    return root;
                }

                e = e.StylingParent;
            }

            return null;
        }

        void IAvaloniaListItemValidator<ILogical>.Validate(ILogical item)
        {
            if (item is null)
            {
                throw new ArgumentException($"Cannot add null to {nameof(LogicalChildren)}.");
            }
        }

        private void ApplyControlTheme()
        {
            if (GetEffectiveTheme() is { } theme)
                ApplyControlTheme(theme, FrameType.Theme);
        }

        private void ApplyTemplatedParentControlTheme()
        {
            if ((TemplatedParent as StyledElement)?.GetEffectiveTheme() is { } parentTheme)
            {
                ApplyControlTheme(parentTheme, FrameType.TemplatedParentTheme);
            }
        }

        private void ApplyControlTheme(ControlTheme theme, FrameType type)

View on GitHub (pinned to 11c5427268)

Solutions

1. Ensure the object added to LogicalChildren is not null; guard the Add call with a null check.
2. Fix the code path that produces a null child (e.g. a converter or template lookup returning null).

When it happens

Trigger: Raised when null is added to a LogicalChildren collection. Defend by null-checking child elements before adding them and filtering nulls from computed child sequences.

Common situations: See trigger scenarios.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/025a43d5d5f12f5c. Report an issue: GitHub.