Fixed null `thisarg` being passed around in config generator

IPA.Loader/Config/Stores/GeneratedStoreImpl/MakeCreator.cs

@@ -368,7 +368,7 @@ namespace IPA.Config.Stores
 
                 foreach (var member in structure)
                 {
-                    EmitSerializeMember(il, member, GetLocal);
+                    EmitSerializeMember(il, member, GetLocal, GetMethodThis);
                     il.Emit(OpCodes.Stloc, valLocal);
                     il.Emit(OpCodes.Ldloc, mapLocal);
                     il.Emit(OpCodes.Ldstr, member.Name);

IPA.Loader/Config/Stores/GeneratedStoreImpl/Serialization.cs

@@ -18,7 +18,7 @@ namespace IPA.Config.Stores
     internal static partial class GeneratedStoreImpl
     {
         // emit takes no args, leaves Value at top of stack
-        private static void EmitSerializeMember(ILGenerator il, SerializedMemberInfo member, GetLocal GetLocal, Action<ILGenerator> thisarg = null)
+        private static void EmitSerializeMember(ILGenerator il, SerializedMemberInfo member, GetLocal GetLocal, Action<ILGenerator> thisarg)
         {
             EmitLoad(il, member, thisarg);