Argument of type 'Expression<Func<Builder_Search.Customer, string?>>' cannot be used for parameter 'keySelector' of type 'Expression<Func<Builder_Search.Customer, string>>' in 'ISpecificationBuilder<Customer> SpecificationBuilderExtensions.Search<Customer>(ISpecificationBuilder<Customer> builder, Expression<Func<Customer, string>> keySelector, string pattern, int group = 1)' due to differences in the nullability of reference types.