Extensions.cs

        public static IEnumerable<TNode> UnfoldTree<TNode>(this TNode root, Func<TNode, IEnumerable<TNode>> getChildren)
        {
            var queue = new Queue<TNode>(new [] { root });

            while (queue.Count > 0)
            {
                var node = queue.Dequeue();
                yield return node;

                foreach (var child in getChildren(node))
                {
                   queue.Enqueue(child);
                }
            }
        }

        public static IEnumerable<DependencyObject> GetChildren(this DependencyObject node)
        {
            int childrenCount = VisualTreeHelper.GetChildrenCount(node);

            for (int childIndex = 0; childIndex < childrenCount; childIndex++)
            {
                yield return VisualTreeHelper.GetChild(node, childIndex);
            }
        }

        private static IEnumerable<DependencyObject> Descendants(DependencyObject node) =>
            node
                .UnfoldTree(GetChildren)
                .Skip(1);

        public static T FirstDescendantOfType<T>(this DependencyObject node) =>
            Descendants(node)
                .OfType<T>()
                .First();

Usage:
var listView = navigationView.FirstDescendantOfType<ListView>();