Issue setting up Singleton

public sealed class RegularPuzzleSingleton : ISingleton
    {
        private const int TrimLength = 13;
        private Dictionary<string, Type> _regularTypes;
        public IRegularPuzzle RegularPuzzle { get; private set; }

        private RegularPuzzleSingleton() 
        {
            GetRegularTypes();
        }

        public static RegularPuzzleSingleton _instance;

        public static RegularPuzzleSingleton GetInstance()
        {
            _instance ??= new RegularPuzzleSingleton();

            return _instance;
        }

        public void GetRegularTypes()
        {
            var interfaceType = typeof(IRegularPuzzle);
            var implementingTypes = AppDomain.CurrentDomain
                                    .GetAssemblies()
                                    .SelectMany(a => a.GetTypes())
                                    .Where(type => interfaceType.IsAssignableFrom(type)
                                            && !type.IsAbstract
                                            && !type.IsInterface);

            foreach (var type in implementingTypes)
            {
                string name = type.Name.ToLower()[..(type.Name.Length - TrimLength)];
                _regularTypes[name] = type;
            }
        }

        public bool IsRegularType(string extension)
        {
            if (_regularTypes.TryGetValue(extension, out Type? regular) && regular != null)
            {
                RegularPuzzle = (IRegularPuzzle)Activator.CreateInstance(regular);
                return true;
            }

            return false;
        }
    }