EF Core Fluent API Null Foreign Key - entity-framework

Question

I have the following tables : Products, Users and ProductApproval. Whenever user create a new product, the ProductApproval will have a new record with the product ID and a null ApprovedByUserId because it is not approve yet. User can have many ProductApproval but Product only can have one ProductApproval.
The structure is like this:
User.cs
public class User
{
public int Id { get; set; }
public string Username { get; set; }
public virtual ICollection<ProductApproval> ProductApprovals { get; set; }
public User()
{
ProductApprovals = new Collection<ProductApproval>();
}
}
Product.cs
public class Product
{
public int Id { get; set; }
public string Name { get; set; }
public virtual ProductApproval ProductApproval { get; set; }
}
ProductApproval.cs
public class ProductApproval
{
public int Id { get; set; }
public virtual Product Product { get; set; }
public int? ProductId { get; set; }
public virtual User User { get ;set; }
public int? ApprovedByUserId { get; set; }
}
DataContext.cs
modelBuilder.Entity<User>()
.HasMany(u => u.ProductApprovals)
.WithOne(pa => pa.User)
.HasForeignKey(pa => pa.ApprovedByUserId)
.IsRequired(false);
modelBuilder.Entity<Product>()
.HasOne(p => p.ProductApproval)
.WithOne(pa => pa.Product)
.HasForeignKey<ProductApproval>(pa => pa.ProductId);
I already declared the foreign key ApprovedByUserId as nullable, but i still get the following error when i insert a record into productapprovals :
MySqlException: Cannot add or update a child row: a foreign key constraint fails (mysite.productapprovals, CONSTRAINT FK_ProductApprovals_Users_ApprovedByUserId FOREIGN KEY (ApprovedByUserId) REFERENCES users (Id) ON DELETE RESTRICT)
Please advice is there any place i did wrong