fix: Write for non-spawned NB only writes 0 netId (fixes: #3399) (#3400)

* Failing test for non-spawned NB writer/reader

* fix: Write for non-spawned NB only writes 0 netId

Instead of writing 0 and component index which does not match what the reader expects (it will stop reading netId is 0)
Fixes #3399

Assets/Mirror/Core/NetworkWriterExtensions.cs

@@ -228,6 +228,22 @@ public static void WriteNetworkBehaviour(this NetworkWriter writer, NetworkBehav
                 writer.WriteUInt(0);
                 return;
             }
+            
+            // users might try to use unspawned / prefab NetworkBehaviours in
+            // rpcs/cmds/syncvars/messages. they would be null on the other
+            // end, and it might not be obvious why. let's make it obvious.
+            // https://github.com/vis2k/Mirror/issues/2060
+            // and more recently https://github.com/MirrorNetworking/Mirror/issues/3399
+            //
+            // => warning (instead of exception) because we also use a warning
+            //    when writing an unspawned NetworkIdentity
+            if (value.netId == 0)
+            {
+                Debug.LogWarning($"Attempted to serialize unspawned NetworkBehaviour: of type {value.GetType()} on GameObject {value.name}. Prefabs and unspawned GameObjects would always be null on the other side. Please spawn it before using it in [SyncVar]s/Rpcs/Cmds/NetworkMessages etc.");
+                writer.WriteUInt(0);
+                return;
+            }
+
             writer.WriteUInt(value.netId);
             writer.WriteByte(value.ComponentIndex);
         }

Assets/Mirror/Tests/Editor/NetworkWriterTest.cs

@@ -1451,6 +1451,25 @@ public void TestNetworkBehaviourNull()
             Assert.That(reader.Position, Is.EqualTo(4), "should read 4 bytes when netid is 0");
         }
 
+        // test for https://github.com/MirrorNetworking/Mirror/issues/3399
+        [Test]
+        public void TestNetworkBehaviourNotSpawned()
+        {
+            CreateNetworked(out _, out _, out RpcNetworkIdentityBehaviour component);
+            NetworkWriter writer = new NetworkWriter();
+            writer.WriteNetworkBehaviour(component);
+
+            byte[] bytes = writer.ToArray();
+
+            Assert.That(bytes.Length, Is.EqualTo(4), "unspawned Networkbehaviour should be 4 bytes long.");
+
+            NetworkReader reader = new NetworkReader(bytes);
+            RpcNetworkIdentityBehaviour actual = reader.ReadNetworkBehaviour<RpcNetworkIdentityBehaviour>();
+            Assert.That(actual, Is.Null, "should read null");
+
+            Assert.That(reader.Position, Is.EqualTo(4), "should read 4 bytes when netid is 0");
+        }
+
         // test to prevent https://github.com/vis2k/Mirror/issues/2972
         [Test]
         public void TestNetworkBehaviourDoesntExistOnClient()