How to implement Thread safe state object (written in one, read in many)?

Question

As I understand it, I need to use ReaderWriterLockSlim from here: https://learn.microsoft.com/en-us/dotnet/api/system.threading.readerwriterlockslim

I did everything like theirs, crashes with System.NullReferenceException:

Object reference not set to an instance of an object.

Updated .... OK, I removed the check for null and added a check for the version of the object in each reading trade. Now works without errors - thanks Peter Duniho

 using System;
    using System.Threading;

    public class Player {
        public int id;
        public override string ToString() {
            return id.ToString();
        }
    }
    public class SlimTest{
        ReaderWriterLockSlim locker = new ReaderWriterLockSlim();
        Random R = new Random();
        Player _leader;
        public Player leader {
            get {
                locker.EnterReadLock();
                try {
                    return _leader;
                }
                finally {
                    locker.ExitReadLock();
                }
            }
        }
        int _id = 0;
        public SlimTest() {
            var main = new Thread(() => {
                while(true) {
                    locker.EnterWriteLock();
                    try {
                        var cf = R.Next(1, 10) % 2;
                        if(cf > 0)
                            _leader = new Player() { id = _id += 1 };
                    }
                    finally {
                        locker.ExitWriteLock();
                    }
                    Thread.Sleep(R.Next(3, 10));
                }
            });
            main.IsBackground = true;
            main.Start();


            for(int i = 0; i < 10;i++) {
                var reader = new Thread(() => {
                    int my_id=0;
                    while(true) {
                        if(leader == null)
                            continue;
                        if(my_id != leader.id) {
                            my_id = leader.id;
                            Console.Write(leader.id);
                        }
                        else
                            Console.Write(".");
                        Thread.Sleep(R.Next(3, 10));
                    }
                 
                });
                reader.IsBackground = true;
                reader.Start();
            }
        }
    }