Oject referrence is not set to instance of object

using System;
 using System.IO;
 using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.ViewEngines;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using AspnetRun.Application.Services;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.AspNetCore.Mvc.Razor;
      
      namespace AspnetRun.Web.Pages.Account
{
public class RenderView : Controller
{
    ServiceCollection serviceCollection = new Microsoft.Extensions.DependencyInjection.ServiceCollection();
    private readonly IRazorViewEngine _razorViewEngine;
    RazorViewEngine  Engine;

private readonly IServiceProvider _serviceProvider ;

public RenderView()
    {
        this._serviceProvider =(IServiceProvider) serviceCollection.BuildServiceProvider();
        _razorViewEngine=Engine;
        serviceCollection.AddRazorPages();
    }
public string RenderPartialViewToString(String ViewPath,string viewName, object model)
{
    
    if (string.IsNullOrEmpty(viewName))
        viewName = ControllerContext.ActionDescriptor.DisplayName;

    ViewData.Model = model;

    using (StringWriter sw = new StringWriter())
    {
         //_serviceProvider
        //var engine =_serviceProvider.GetService(typeof(ICompositeViewEngine)) as ICompositeViewEngine;
         // Resolver.GetService(typeof(ICompositeViewEngine)) as ICompositeViewEngine;
        var engine = _serviceProvider.GetService(typeof(IRazorViewEngine)) as IRazorViewEngine;
         engine=Engine;
       
        ViewEngineResult viewResult = engine.FindView(ControllerContext, viewName, false);//.GetView("~/",ViewPath,false);//.FindView(ControllerContext, viewName, false);

        ViewContext viewContext = new ViewContext(
            ControllerContext,
            viewResult.View,
            ViewData,
            TempData,
            sw,
            new HtmlHelperOptions() //Added this parameter in
        );

        //Everything is async now!
        var t = viewResult.View.RenderAsync(viewContext);
        t.Wait();

        return sw.GetStringBuilder().ToString();
    }
}

}
}