Fixing a nullreference exception

Also util files to start only "external containers"

docker-compose-external.override.yml

@@ -0,0 +1,17 @@
+#
+# docker-compose.override.yml is used to set up local configuration environment
+# Things like the external ports to use or secrets/passwords depend on the 
+# specific deployment environment you might be using.
+# Further details and docs: https://docs.docker.com/compose/extends/ 
+#
+
+version: '2'
+
+services:
+
+  sql.data:
+    environment:
+      - SA_PASSWORD=Pass@word
+      - ACCEPT_EULA=Y
+    ports:
+      - "5433:1433"

docker-compose-external.yml

@@ -0,0 +1,16 @@
+#
+# docker-compose.yml is used to set up the base config per container to be deployed
+# Take into account that when deploying, this base configuration is merged with the 
+# configuration-per-environment specified at the docker-compose.override.yml
+# Further details and docs: https://docs.docker.com/compose/extends/ 
+#
+version: '2'
+
+services:
+  sql.data:
+    image: microsoft/mssql-server-linux
+
+  basket.data:
+    image: redis
+    ports:
+      - "6379:6379"

src/Services/Ordering/Ordering.Domain/AggregatesModel/BuyerAggregate/Buyer.cs

@@ -1,5 +1,6 @@
 using Microsoft.eShopOnContainers.Services.Ordering.Domain.Seedwork;
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 
@@ -12,11 +13,13 @@ namespace Microsoft.eShopOnContainers.Services.Ordering.Domain.AggregatesModel.B
 
         private List<PaymentMethod> _paymentMethods;
 
-        public IEnumerable<PaymentMethod> PaymentMethods => _paymentMethods?.AsReadOnly();
+        public IEnumerable<PaymentMethod> PaymentMethods => _paymentMethods.AsReadOnly();
 
-        protected Buyer() { }
+        protected Buyer() {
+            _paymentMethods = new List<PaymentMethod>();
+        }
 
-        public Buyer(string identity)
+        public Buyer(string identity) : this()
         {
             if (String.IsNullOrWhiteSpace(identity))
             {
@@ -25,7 +28,6 @@ namespace Microsoft.eShopOnContainers.Services.Ordering.Domain.AggregatesModel.B
 
             IdentityGuid = identity;
 
-            _paymentMethods = new List<PaymentMethod>();
         }
 
         public PaymentMethod AddPaymentMethod(int cardTypeId, string alias, string cardNumber, string securityNumber, string cardHolderName, DateTime expiration)

start-external.ps1

@@ -0,0 +1 @@
+docker-compose -f docker-compose-external.yml -f docker-compose-external.override.yml up