public void Add([NotNull] PropertyChangedEventHandler handler)
        {
            if (handler == null)
            {
                throw new ArgumentNullException(nameof(handler));
            }

            ThrowExceptionIfDisposed();
            _bag.Add(handler);
        }
 public void Add(PropertyChangedEventHandler handler)
 {
     ThrowExceptionIfDisposed();
     _bag.Add(handler);
 }