An abstraction for accessing singleton instances in C# (made for the use in Unity3D)

SingletonProvider.cs

using System;
using System.Collections.Generic;

namespace FMPUtils
{
    public static class SingletonProvider
    {
        private static readonly Dictionary<Type, Func<object>> instanceAccessors = new Dictionary<Type, Func<object>>();

        public static void Clear()
        {
            instanceAccessors.Clear();
        }

        public static void AddSingleton<T>(Func<T> accessorMethod, bool overwriteIfAccessorExists = true) where T : class
        {
            if (accessorMethod == null) return;
            if (!overwriteIfAccessorExists && instanceAccessors.ContainsKey(typeof(T))) return;

            instanceAccessors[typeof(T)] = accessorMethod;
        }

        public static T Get<T>() where T : class
        {
            if (instanceAccessors.TryGetValue(typeof(T), out Func<object> accessorMethod))
            {
                return accessorMethod?.Invoke() as T;
            }
            return default(T);
        }
    }
}

SingletonProviderUsageExample.cs

 public class SingletonProviderUsageExample
    {
        public interface IMessenger
        {
            void SendMessage();
        }

        public class Messenger : IMessenger
        {
            public static Messenger Instance => instance;

            private static readonly Messenger instance = new Messenger();

            private Messenger() { }

            public void SendMessage() { }
        }

        void DemonstrateMethods()
        {
            // Register accessor method for singleton:
            SingletonProvider.AddSingleton<IMessenger>(() => Messenger.Instance);
            // Retrieve singleton instance for interface type:
            var messenger = SingletonProvider.Get<IMessenger>();
        }
    }

Note: I was told that the concept of this DI container is usually called a "ServiceProvider", but even if this doesn't just work for Singletons I'll keep the existing name because I am under the impression that people that use Unity3D might rather look for the term "Singleton.." than "ServiceProvider"