I had the following working until i upgraded to 7.52. this is a composition named product that has a link for external site. after the upgrade it is like Umbraco can not see Nested Content.

@{
    var items = Model.GetPropertyValue<IEnumerable<IPublishedContent>>("product");

    <div class="row">
        @foreach (var item in items)
        {
            var widgetLink = item.GetPropertyValue<UrlPicker>("productLink");
            var imageId = item.GetPropertyValue<int>("productImage");
            var image = Umbraco.TypedMedia(item.GetPropertyValue("productImage"));

            if (widgetLink != null && widgetLink.Meta != null && widgetLink.Type != null)
            {
                var linkTarget = widgetLink.Meta.NewWindow ? "_blank" : null;
                var linkUrl = widgetLink.Url;

                string linkText = null;
                if (widgetLink.Meta.Title != null)
                {
                    linkText = widgetLink.Meta.Title;
                }

                if (widgetLink.Type == UrlPicker.UrlPickerTypes.Url)
                {
                    if (!string.IsNullOrEmpty(linkUrl) && !(linkUrl.StartsWith("http://") && !(linkUrl.StartsWith("https://"))))
                    {
                        linkUrl = "http://" + linkUrl;
                    }
                }

                if (!string.IsNullOrEmpty(linkUrl))
                {
                    <div class="col-lg-3">

                        <a href="@linkUrl" target="@linkTarget" title="@linkText">
                            @if (item.HasValue("productImage"))
                            {
                                <img class="img-responsive" src="@image.Url" width="270" height="100" alt="@item.Name"/>
                            }
                        </a>
                    </div>
                }
            }
        }
    </div>
}