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