On Page creation of block does not show custom error message when saving

Hi,

I've a custom validator (MinMaxAttribute) added to a block property.

When I create a new block of that type and save it for the first time, the error message that it needs at least x items is not showing and the block is not added to the ContentArea because of missing rules.

When I create a block following the rules and after saving this block, it will be added.
Deleting one item so it will not pass the rules, the error is shown.
But from here you can publish this block instead of saving this block by creation.

How can I fix this?

My property:

[MinMax(Min = 4, Max = 10)]
[AllowedTypes(new[] { typeof(OnPageNavigationItem) })]
[Display(Name = "Items", Description = "Min. 4, Max. 10 items", Order = 300)]
public virtual ContentArea Items { get; set; }

My custom error attribute:

[AttributeUsage(AttributeTargets.Property)]
    public class MinMaxAttribute : ValidationAttribute
    {
        private static readonly IContentLoader _contentLoader = ServiceLocator.Current.GetInstance<IContentLoader>();
        public int Min
        {
            get;
            set;
        }
        public int Max
        {
            get;
            set;
        }
        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            if ((value != null) && !(value is ContentArea) && !(value is LinkItemCollection))
            {
                throw new ValidationException("MinMaxAttribute is intended only for use with ContentArea or LinkItemCollection properties");
            }
            var contentArea = value as ContentArea;
            var linkItemCollection = value as LinkItemCollection;
            var minError = false;
            var maxError = false;
            var min = Min;
            var max = Max;
            if (contentArea != null || linkItemCollection != null)
            {
                var items = contentArea?.Items.Select(c => c.ContentLink).Count() ?? linkItemCollection.Count;
                if (min > 0 && items < min)
                {
                    minError = true;
                }
                if (max > 0 && items > max)
                {
                    maxError = true;
                }
            }
            else if (min > 0)
            {
                minError = true;
            }
            if (minError || maxError)
            {
                if (max > 0 && min > 0)
                {
                    if (min == max)
                    {
                        ErrorMessage = string.Format("Number of items for property '{0}' should be {1}", validationContext.DisplayName, min);
                    }
                    else
                    {
                        ErrorMessage = string.Format("Number of items for property '{0}' should be between {1} and {2}", validationContext.DisplayName, min, max);
                    }
                }
                else if (max > 0)
                {
                    ErrorMessage = string.Format("Number of items for property '{0}' should be maximum {1}", validationContext.DisplayName, max);
                }
                else
                {
                    ErrorMessage = string.Format("Number of items for property '{0}' should be minimum {1}", validationContext.DisplayName, min);
                }
                return new ValidationResult(ErrorMessage, new[] { validationContext.MemberName });
            }
            return null;
        }
    }

Thanks in advance!

Cheers,
Sander