fix null lookup bug in message manager

encompass-cs/MessageManager.cs

@@ -28,7 +28,9 @@ namespace Encompass
 
         internal IEnumerable<TMessage> GetMessagesByType<TMessage>() where TMessage : struct, IMessage
         {
-            return messageTypeToMessages[typeof(TMessage)].Cast<TMessage>();
+            return messageTypeToMessages.ContainsKey(typeof(TMessage)) ?
+                messageTypeToMessages[typeof(TMessage)].Cast<TMessage>() :
+                Enumerable.Empty<TMessage>();
         }
     }
 }

test/EngineTest.cs

@@ -227,6 +227,29 @@ namespace Tests
             }
         }
 
+        static IEnumerable<MockMessage> emptyReadMessagesResult;
+        [Reads(typeof(MockMessage))]
+        class ReadMessagesWhenNoneExistEngine : Engine
+        {
+            public override void Update(float dt)
+            {
+                emptyReadMessagesResult = ReadMessages<MockMessage>();
+            }
+        }
+
+        [Test]
+        public void ReadMessagesWhenNoneHaveBeenEmitted()
+        {
+            var worldBuilder = new WorldBuilder();
+            worldBuilder.AddEngine<ReadMessagesWhenNoneExistEngine>();
+
+            var world = worldBuilder.Build();
+
+            world.Update(0.01f);
+
+            emptyReadMessagesResult.Should().BeEmpty();
+        }
+
         [Test]
         public void EmitUndeclaredMessage()
         {