C#Office 365 Sharepoint文件下载-ServerObjectNullReferenceException

我试图从SharePoint列表获取文件URL列表，但每次尝试从该文件访问属性时，都会遇到ServerObjectNullReferenceException

似乎找不到从ClientContext或通过ListItemCollection本身加载此服务器对象的方法

我错过了什么

            SecureString passWord = new SecureString();
            foreach (char c in "password".ToCharArray()) passWord.AppendChar(c);

            ClientContext clientContext = new ClientContext("url");
            context.Credentials = new SharePointOnlineCredentials("username", passWord);

            var list = clientContext.Web.GetList("/Files/Apps/");

            var listItems = list.GetItems(new CamlQuery());

            clientContext.Load(listItems,
                items => items.Include(
                item => item.File));

            clientContext.ExecuteQuery();

            // Any time I access item.File I get the ServerObjectNullReferenceException
            foreach (var item in listItems)
            {
                string test = String.Format("{0}", item.File.ServerRelativeUrl);
            }