Null Reference Exception in navigation property using Linq query in ASP.Net

In an action method of mvc project i get error of null reference

public async Task<IActionResult> List()
{
    var solvency = await _solvencyRepository.GetClientiAsync();
    var orders = await _ordersRepository.GetOrders();          
    var userOrders = (from o in orders
                      join s in solvency on o.Code equals s.Code into os
                      from x in os.DefaultIfEmpty()
                      orderby o.Date descending
                      let block = ...
                      select new
                      {                                
                          Codcli = o.Codcli,
                          CustomerName = o.Customer.Name,
                          ...
                                  
                      }).ToList();
    return Ok(userOrders);
}

The error refers to CustomerName

how can solve?