added code from unity

Runtime/PopupCategoryJsonConverter.cs

@@ -0,0 +1,15 @@
+using System;
+using Newtonsoft.Json;
+
+namespace Jovian.PopupSystem {
+    public sealed class PopupCategoryJsonConverter : JsonConverter<PopupCategory> {
+        public override void WriteJson(JsonWriter writer, PopupCategory value, JsonSerializer serializer) {
+            writer.WriteValue(value.Id);
+        }
+
+        public override PopupCategory ReadJson(JsonReader reader, Type objectType, PopupCategory existingValue, bool hasExistingValue, JsonSerializer serializer) {
+            var id = reader.Value as string;
+            return new PopupCategory(id);
+        }
+    }
+}