Reactor/Reactor/PluginSingleton.cs

using System;
using System.Linq;
using BepInEx.IL2CPP;

namespace Reactor
{
    public class PluginSingleton<T> where T : BasePlugin
    {
        private static T _instance;

        public static T Instance
        {
            get
            {
                if (_instance == null)
                {
                    _instance = Preloader.Chainloader.Plugins.Values.Select(x => x.Instance).OfType<T>().Single();
                }

                return _instance;
            }

            set
            {
                if (_instance != null)
                {
                    throw new Exception($"Instance for {typeof(T).FullName} is already set");
                }

                _instance = value;
            }
        }
    }
}