initialitze the list in constructor

src/Services/Marketing/Marketing.API/Model/Campaing.cs

@@ -15,6 +15,12 @@
 
         public string Url { get; set; }
 
-        public ICollection<Rule> Rules { get; set; }
+        public List<Rule> Rules { get; set; }
+
+
+        public Campaign()
+        {
+            Rules = new List<Rule>();
+        }
     }
 }

src/Services/Marketing/Marketing.API/Model/Rule.cs

@@ -1,4 +1,4 @@
-using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations.Schema;
 
 namespace Microsoft.eShopOnContainers.Services.Marketing.API.Model
 {
@@ -26,4 +26,4 @@ namespace Microsoft.eShopOnContainers.Services.Marketing.API.Model
     {
         public int LocationId { get; set; }
     }
-}
+}