DependencyService.Get<ILogManager>().GetLog() System.NullReferenceException: 'Object reference not set to an instance of an object'

Question

I am trying to execute Xamarin App.

I can build and deploy solutions on both Android and iOS devices. But when I am Debugging/running iOS App I am receiving an error **System.NullReferenceException:** 'Object reference not set to an instance of an object' on
private static ILogger logger = DependencyService.Get<ILogManager>().GetLog(); line

I have installed the latest version of the Nlog NuGet Package.

My ILogManager file is

namespace VolMobile.AppData.Interfaces
{
    public interface ILogManager
    {
        ILogger GetLog([System.Runtime.CompilerServices.CallerFilePath]string callerFilePath = "");

        void Reload();

        void DeleteLog();
    }
}

How can I resolve this issue?

update

My NLogManager iOS file

using System;
using System.Collections.Generic;
using Xamarin.Forms;
using System.IO;
using NLog;
using NLog.Config;
using NLog.Targets;
using VolMobile.AppData.Interfaces;
using VolMobile.iOS.Logging;

[assembly: Dependency(typeof(NLogManager))]
namespace VolMobile.iOS.Logging
{
    public class NLogManager : ILogManager
    {
        string logFile;
        LoggingConfiguration config;


        public NLogManager()
        {

            Reload();

        }



        public void Reload()
        {
            config = new LoggingConfiguration();

            var consoleTarget = new ConsoleTarget();
            config.AddTarget("console", consoleTarget);

            var consoleRule = new LoggingRule("*", LogLevel.Trace, consoleTarget);
            config.LoggingRules.Add(consoleRule);
            //File logging level
            LogLevel llSetting = LogLevel.Off;
            IEnumerable<LogLevel> sysLevels = LogLevel.AllLevels;

            //default to trace at startup
            string currentLogLevel = "Trace";

            //load the app state if available
            if (App.AppState != null)
                currentLogLevel = App.AppState.AppSettings.LogLevel;// AppData.AppData.LogLevel;
            foreach (LogLevel ll in sysLevels)
            {

                if (currentLogLevel == ll.Name)
                {
                    llSetting = ll;
                }
            }

            var fileTarget = new FileTarget();
            string folder = System.Environment.GetFolderPath(System.Environment.SpecialFolder.MyDocuments); //android.os.environment is the other option
            fileTarget.FileName = Path.Combine(folder, "Log.txt");

            fileTarget.Layout = "${longdate}|${level:uppercase=true}|${callsite}|${appdomain}|${logger}|${threadid}|${message}|{exception:format=tostring}";

            config.AddTarget("file", fileTarget);
            logFile = Path.Combine(folder, "Log.txt");



            var fileRule = new LoggingRule("*", llSetting, fileTarget);//LogLevel.Warn
            config.LoggingRules.Add(fileRule);



            LogManager.Configuration = config;
        }

        public void DeleteLog()
        {
            File.Delete(logFile);
        }

        // Services.Logging.ILogger
        //AppData.Interfaces.ILogger
        //public NLogLogger GetLog([System.Runtime.CompilerServices.CallerFilePath] string callerFilePath = "")
        //{
        //    string fileName = callerFilePath;

        //    if (fileName.Contains("/"))
        //    {
        //        fileName = fileName.Substring(fileName.LastIndexOf("/", StringComparison.CurrentCultureIgnoreCase) + 1);
        //    }

        //    var logger = LogManager.GetLogger(fileName);
        //    return new NLogLogger(logger, logFile);

        //}

        public AppData.Interfaces.ILogger GetLog([System.Runtime.CompilerServices.CallerFilePath] string callerFilePath = "")
        {
            string fileName = callerFilePath;

            if (fileName.Contains("/"))
            {
                fileName = fileName.Substring(fileName.LastIndexOf("/", StringComparison.CurrentCultureIgnoreCase) + 1);
            }

            var logger = LogManager.GetLogger(fileName);
            return new NLogLogger(logger, logFile);
        }
    }
}

The app is working perfectly fine on iOS, Andriod simulators, and Android Device only not on iOS devices.