Fix incorrect JsonAssetReference<T> serialization

#2774

Source/Engine/Content/JsonAssetReference.cs

@@ -12,6 +12,7 @@ namespace FlaxEngine
 #if FLAX_EDITOR
     [CustomEditor(typeof(FlaxEditor.CustomEditors.Editors.AssetRefEditor))]
 #endif
+    [Newtonsoft.Json.JsonConverter(typeof(Json.JsonAssetReferenceConverter))]
     public struct JsonAssetReference<T> : IComparable, IComparable<JsonAssetReference<T>>, IEquatable<JsonAssetReference<T>>
     {
         /// <summary>

Source/Engine/Serialization/JsonConverters.cs

@@ -429,6 +429,60 @@ namespace FlaxEngine.Json
         }
     }
 
+    internal sealed class JsonAssetReferenceConverter : JsonConverter
+    {
+        /// <inheritdoc />
+        public override unsafe void WriteJson(JsonWriter writer, object value, Newtonsoft.Json.JsonSerializer serializer)
+        {
+            var asset = (JsonAsset)value.GetType().GetField("Asset").GetValue(value);
+            var id = asset?.ID ?? Guid.Empty;
+            writer.WriteValue(JsonSerializer.GetStringID(&id));
+        }
+
+        /// <inheritdoc />
+        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, Newtonsoft.Json.JsonSerializer serializer)
+        {
+            var result = Activator.CreateInstance(objectType);
+            if (reader.TokenType == JsonToken.String)
+            {
+                JsonSerializer.ParseID((string)reader.Value, out var id);
+                var asset = Content.LoadAsync<JsonAsset>(id);
+                objectType.GetField("Asset").SetValue(result, asset);
+            }
+            else if (reader.TokenType == JsonToken.StartObject)
+            {
+                // [Deprecated on 26.07.2024, expires on 26.07.2026]
+                while (reader.Read() && reader.TokenType != JsonToken.EndObject)
+                {
+                    switch (reader.TokenType)
+                    {
+                    case JsonToken.PropertyName:
+                    {
+                        var propertyName = (string)reader.Value;
+                        reader.Read();
+                        if (propertyName == "Asset" && reader.TokenType == JsonToken.String)
+                        {
+                            JsonSerializer.ParseID((string)reader.Value, out var id);
+                            var asset = Content.LoadAsync<JsonAsset>(id);
+                            objectType.GetField("Asset").SetValue(result, asset);
+                        }
+
+                        break;
+                    }
+                    }
+                }
+            }
+
+            return result;
+        }
+
+        /// <inheritdoc />
+        public override bool CanConvert(Type objectType)
+        {
+            return objectType.Name.StartsWith("JsonAssetReference");
+        }
+    }
+
     /*
     /// <summary>
     /// Serialize Guid values using `N` format