Server class for kestrel web server

Server.cs

using Microsoft.AspNetCore.Hosting;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading.Tasks;

namespace WebServer.Main
{
    public class Server
    {
        private WebHostBuilder _serverBuilder = null;
        private IWebHost _server = null;
        public Server(string path, string port = "1515", string webroot = "")
        {

            if (webroot == null)
                throw new ArgumentNullException("webroot");

            if (String.IsNullOrEmpty(path))
                path = Directory.GetCurrentDirectory();

            if (String.IsNullOrEmpty(port))
                port = "5000";

            _serverBuilder = new WebHostBuilder();

            _server = _serverBuilder
                .UseKestrel()
                .UseContentRoot(path)
                .UseWebRoot(webroot)
                .UseStartup<Startup>()
                .UseUrls($"http://localhost:{port}")
                .Build();
        }

        public void Start()
        {
            _server.Run();
        }
    }
}