breaking: Users must initialize synclists (#391) (#2330)

* Users must initialize syncobjects (#391)

Previously we initialized syncobjects, so this is valid:
```cs
public class Pepe : NetworkBehavior {

    public SyncList<int> mylist;
}
```

With this change,  users must initialize their own fields:
```cs
public class Pepe : NetworkBehavior {

    public SyncList<int> mylist = new SyncList<int>();
}
```

BREAKING CHANGE: You must initialize all your SyncLists

* Add null check

* This is no longer a weaver error

* Update Assets/Mirror/Runtime/NetworkBehaviour.cs

Co-authored-by: James Frowen <jamesfrowendev@gmail.com>

* Remove unnecesary using

Co-authored-by: James Frowen <jamesfrowendev@gmail.com>

Assets/Mirror/Editor/Weaver/Processors/SyncObjectInitializer.cs

@@ -1,4 +1,3 @@
-using System.Linq;
 using Mono.CecilX;
 using Mono.CecilX.Cil;
 
@@ -8,56 +7,10 @@ public static class SyncObjectInitializer
     {
         public static void GenerateSyncObjectInitializer(ILProcessor worker, FieldDefinition fd)
         {
-            // call syncobject constructor
-            GenerateSyncObjectInstanceInitializer(worker, fd);
-
             // register syncobject in network behaviour
             GenerateSyncObjectRegistration(worker, fd);
         }
 
-        // generates 'syncListInt = new SyncListInt()' if user didn't do that yet
-        static void GenerateSyncObjectInstanceInitializer(ILProcessor worker, FieldDefinition fd)
-        {
-            // check the ctor's instructions for an Stfld op-code for this specific sync list field.
-            foreach (Instruction ins in worker.Body.Instructions)
-            {
-                if (ins.OpCode.Code == Code.Stfld)
-                {
-                    FieldDefinition field = (FieldDefinition)ins.Operand;
-                    if (field.DeclaringType == fd.DeclaringType && field.Name == fd.Name)
-                    {
-                        // Already initialized by the user in the field definition, e.g:
-                        // public SyncListInt Foo = new SyncListInt();
-                        return;
-                    }
-                }
-            }
-
-            // Not initialized by the user in the field definition, e.g:
-            // public SyncListInt Foo;
-
-            TypeDefinition fieldType = fd.FieldType.Resolve();
-            // find ctor with no parameters
-            MethodDefinition ctor = fieldType.Methods.FirstOrDefault(x => x.Name == ".ctor" && !x.HasParameters);
-            if (ctor == null)
-            {
-                Weaver.Error($"Can not initialize field {fd.Name} because no default constructor was found. Manually initialize the field (call the constructor) or add constructor without Parameter", fd);
-                return;
-            }
-            MethodReference objectConstructor = Weaver.CurrentAssembly.MainModule.ImportReference(ctor);
-
-            // if is SyncList<int> instead of SyncListInt then we need to make the ctor generic 
-            if (fd.FieldType.IsGenericInstance)
-            {
-                GenericInstanceType genericInstance = (GenericInstanceType)fd.FieldType;
-                objectConstructor = objectConstructor.MakeHostInstanceGeneric(genericInstance);
-            }
-
-            worker.Append(worker.Create(OpCodes.Ldarg_0));
-            worker.Append(worker.Create(OpCodes.Newobj, objectConstructor));
-            worker.Append(worker.Create(OpCodes.Stfld, fd));
-        }
-
         public static bool ImplementsSyncObject(TypeReference typeRef)
         {
             try

Assets/Mirror/Runtime/NetworkBehaviour.cs

@@ -154,7 +154,10 @@ public int ComponentIndex
         // We collect all of them and we synchronize them with OnSerialize/OnDeserialize
         protected void InitSyncObject(SyncObject syncObject)
         {
-            syncObjects.Add(syncObject);
+            if (syncObject == null)
+                Debug.LogError("Uninitialized SyncObject. Manually call the constructor on your SyncList, SyncSet or SyncDictionary", this);
+            else
+                syncObjects.Add(syncObject);
         }
 
         #region Commands

Assets/Mirror/Tests/Editor/Weaver/WeaverSyncListTests.cs

@@ -43,8 +43,7 @@ public void SyncListInheritance()
         [Test]
         public void SyncListMissingParamlessCtor()
         {
-            HasError("Can not initialize field Foo because no default constructor was found. Manually initialize the field (call the constructor) or add constructor without Parameter",
+            IsSuccess();
-                "WeaverSyncListTests.SyncListMissingParamlessCtor.SyncListMissingParamlessCtor/SyncListString2 WeaverSyncListTests.SyncListMissingParamlessCtor.SyncListMissingParamlessCtor::Foo");
         }
 
         [Test]