如何使我的类上的自定义事件消费成为可选？

当我注释掉fm.OnLoaded下面一行，它给我一个错误，OnLoaded 为空。

如何使我的类的调用者可以选择是否像 .NET 类/事件一样使用该事件？

using System;
using System.Windows;

namespace TestEventLoaded8282
{
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();

            FileManager fm = new FileManager();
            //fm.OnLoaded += new FileManager.LoadedHandler(fm_OnLoaded);
            fm.Load();
        }

        void fm_OnLoaded(object obj, FileManagerArgs args)
        {
            Console.WriteLine("the file manager is loaded: " + args.Message); 
        }
    }

    public class FileManager
    {
        public string Name { get; set; }

        public delegate void LoadedHandler(object obj, FileManagerArgs args);
        public event LoadedHandler OnLoaded;

        public FileManager()
        {}

        public void Load()
        {
            Name = "this is the test file manager";
            OnLoaded(this, new FileManagerArgs("no errors"));
        }
    }

    public class FileManagerArgs : EventArgs
    {
        public string Message { get; set; }

        public FileManagerArgs(string message)
        {
            Message = message;
        }
    }
}

if (OnLoaded != null) {
    OnLoaded(this, new FileManagerArgs("no errors"));
}