Behavior to capture the change of source collection.

ItemsControlBehavior.cs

using System;
using System.Collections;
using System.Collections.Specialized;
using System.ComponentModel;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using Microsoft.Xaml.Behaviors;

public class ItemsControlBehavior : Behavior<ItemsControl>
{
    protected override void OnAttached()
    {
        base.OnAttached();

        var dpd = DependencyPropertyDescriptor.FromProperty(ItemsControl.ItemsSourceProperty, typeof(ItemsControl));
        dpd?.AddValueChanged(AssociatedObject, OnItemsSourceChanged);

        AddSourceCollection();
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();

        var dpd = DependencyPropertyDescriptor.FromProperty(ItemsControl.ItemsSourceProperty, typeof(ItemsControl));
        dpd?.RemoveValueChanged(AssociatedObject, OnItemsSourceChanged);

        RemoveSourceCollection();
    }

    private void OnItemsSourceChanged(object? sender, EventArgs e)
    {
        RemoveSourceCollection();
        AddSourceCollection();
    }

    private INotifyCollectionChanged? _collection;

    private void AddSourceCollection()
    {
        _collection = AssociatedObject.ItemsSource as INotifyCollectionChanged;
        if (_collection is not null)
        {
            _collection.CollectionChanged += OnItemsSourceCollectionChanged;
        }
        Apply();
    }

    private void RemoveSourceCollection()
    {
        if (_collection is not null)
        {
            _collection.CollectionChanged -= OnItemsSourceCollectionChanged;
        }
        _collection = null;
    }

    private void OnItemsSourceCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
    {
        Apply();
    }

    private void Apply()
    {
        AssociatedObject.IsEnabled = _collection is ICollection { Count: > 1 };

        if (AssociatedObject is Selector selector)
        {
            selector.SelectedIndex = 0;
        }
    }
}