title: why is the list<> property of the object is null
body: <p>Edit: I am not asking about what System.NullReferenceException is. I'm trying to understand why the list property of the object is null and how to prevent it.</p>
  
  <p>When i try to add a new item to my customer's orders list, i am getting System.NullReferenceException. I thought about making list property static but i am not sure that is the correct way.
  What is the proper way of defining classes in this case?</p>
  
  <pre><code>        using(var northwindContext = new NorthwindContext())
          {
              Customer customer = northwindContext.Customers.Find("ENGIN");
              customer.Orders.Add(
                  new Order { 
                      OrderId = 1, 
                      OrderDate = DateTime.Now, 
                      ShipCity = "Ankara", 
                      ShipCountry = "Türkiye" 
                  });
  
              northwindContext.SaveChanges();
  
          }
  
  
  public class Customer
  {
      public string CustomerID { get; set; }
  
      public string ContactName { get; set; }
  
      public string CompanyName { get; set; }
  
      public string City { get; set; }
  
      public string Country { get; set; }
  
  
      public List&lt;Order&gt; Orders { get; set; }
  }
  
  public class Order
  {
      public int OrderId { get; set; }
  
      public string CustomerID { get; set; }
  
      public string ShipCity { get; set; }
  
      public string ShipCountry { get; set; }
  
      public DateTime OrderDate { get; set; }
  
      public Customer Customer { get; set; }
  
  
  }
  </code></pre>