EF Core: Circular entity reference

I would say that it takes a lot of time to get to know ASP.NET Core to understand how to achieve things then previous versions with webforms, but I understand that ASP.NET Core is bigger and you are able to build more complex solutions.

I'm quit new to ASP.NET Core and I'm trying to understand EF Core and related data. I'm using https://docs.microsoft.com/en-us/aspnet/core/data/ef-mvc/intro to learn the basics and create my first ASP.NET Core application.

I have a Entity "Standard" that can have multiple Forms (Form entity). The enitities share a couple of same properties so I've made them both inherit from a master class called MasterDocument. Previously called Document. 

Standard:

namespace Skjemabasen.Models.Document
{
    public class Standard : MasterDocument
    {
        [Display(Name = "Kategori")]
        public virtual Category Category { get; set; }
        [Display(Name = "Dokumenter")]
        public ICollection<Form> Forms { get; set; }
    }
}

Form:

    public class Form : MasterDocument
    {
        public Format Format { get; set; }
        public virtual Member Assignee { get; set; }
        public String Status { get; set; }
        [ForeignKey("Standard")]
        public int StandardId { get; set; }
        public Standard Standard { get; set; }
        public ICollection<Customer.Subscription> Subscribers { get; set; }
    }

MasterDocument:

namespace Skjemabasen.Models.Document
{
    public class MasterDocument : IDocument
    {
        public int ID { get; set; }
        [Required]
        [Display(Name = "EStandard")]
        [StringLength(50)]
        public string EStandard { get; set; }
        [Required]
        [Column("Betegnelse")]
        [Display(Name = "Betegnelse")]
        [StringLength(60)]
        public string Betegnelse { get; set; }
        [Display(Name = "Kommentar")]
        public string Comment { get; set; }
    }
}

I understand that this can cause circular request or circular deletion so I inserted a deletebehaviour.Restrict on Standard:

            modelBuilder.Entity<Standard>()
                .HasOne(d => d.Forms)
                .WithMany()
                .OnDelete(DeleteBehavior.Restrict);

My complete context class:

namespace Skjemabasen.Data
{
    public class SkjemabasenContext : DbContext
    {
        public SkjemabasenContext(DbContextOptions<SkjemabasenContext> options) :base(options)
        {

        }

        public DbSet<Member> Members { get; set; }
        public DbSet<Category> Categories { get; set; }
        public DbSet<Standard> Standards { get; set; }
        public DbSet<Form> Forms { get; set; }
        public DbSet<Customer> Customers { get; set; }
        public DbSet<Revision> Revisions { get; set; }
        public DbSet<Subscription> Subscriptions { get; set; }
        public DbSet<MasterDocument> Documents { get; set; }

        public IQueryable<Customer> CurrentCustomers
        {
            get { return Customers.Where(c => c.Inactive == false); }
        }

        public IQueryable<Customer> InActiveCustomers
        {
            get { return Customers.Where(c => c.Inactive == true); }
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {

            modelBuilder.Entity<Member>().ToTable("Member");
            modelBuilder.Entity<Category>().ToTable("Category");
            modelBuilder.Entity<Standard>().ToTable("Standard");
            modelBuilder.Entity<Form>().ToTable("Form");
            modelBuilder.Entity<Customer>().ToTable("Customer");
            modelBuilder.Entity<Revision>().ToTable("Revision");
            modelBuilder.Entity<Subscription>().ToTable("Subscription");
            modelBuilder.Entity<MasterDocument>().ToTable("Document");


            modelBuilder.Entity<Standard>()
                .HasOne(d => d.Forms)
                .WithMany()
                .OnDelete(DeleteBehavior.Restrict);
        }
    }
}

When I try to run the application I get the error:

System.ArgumentException: 'The entity type 'System.Collections.Generic.ICollection`1[Skjemabasen.Models.Document.Form]' provided for the argument 'clrType' must be a reference type.'

Because all Forms must have a parent Standard and both 'Standard' and 'Form' inherits from MasterDocument, I understand that ASP.NET Core warns about circular deletion, but I'm not sure how to achieve this. The error says something about ICollection of 'Forms' not being a reference type. Is something missing in 'Standard' related to the relation between and 'Form'.  Based on https://docs.microsoft.com/en-us/aspnet/core/data/ef-mvc/intro I can't figure out what I'm missing here.

In advance thank you very much for helping me understand ASP.NET Core so I can create more cool web applicatoins. Thainks!

Jon Haakon Ariansen