Fix parsing via typetree of old Material assets

- Fixed parsing via typetree of Material assets from Unity version <5.6

AssetStudio/AssetsManager.cs

@@ -515,7 +515,7 @@ namespace AssetStudio
 
             var jsonOptions = new JsonSerializerOptions
             {
-                Converters = { new JsonConverterHelper.ByteArrayConverter(), new JsonConverterHelper.PPtrConverter() },
+                Converters = { new JsonConverterHelper.ByteArrayConverter(), new JsonConverterHelper.PPtrConverter(), new JsonConverterHelper.KVPConverter() },
                 NumberHandling = JsonNumberHandling.AllowNamedFloatingPointLiterals,
                 PropertyNameCaseInsensitive = true,
                 IncludeFields = true,

AssetStudio/JsonConverterHelpers/KVPConverter.cs

@@ -0,0 +1,53 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace AssetStudio
+{
+    public static partial class JsonConverterHelper
+    {
+        public class KVPConverter : JsonConverterFactory
+        {
+            public override bool CanConvert(Type typeToConvert)
+            {
+                if (!typeToConvert.IsGenericType)
+                    return false;
+
+                var generic = typeToConvert.GetGenericTypeDefinition();
+                return generic == typeof(KeyValuePair<,>);
+            }
+
+            public override JsonConverter CreateConverter(Type type, JsonSerializerOptions options)
+            {
+                var kvpArgs = type.GetGenericArguments();
+                return (JsonConverter)Activator.CreateInstance(typeof(KVPConverter<,>).MakeGenericType(kvpArgs));
+            }
+        }
+
+        private class KVPConverter<TKey, TValue> : JsonConverter<KeyValuePair<TKey, TValue>>
+        {
+            public override KeyValuePair<TKey, TValue> Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+            {
+                //startKvpObject
+                reader.Read(); //propName
+                reader.Read(); //keyType
+                var key = reader.TokenType == JsonTokenType.StartObject
+                    ? JsonSerializer.Deserialize<Dictionary<string, TKey>>(ref reader).Values.First()
+                    : JsonSerializer.Deserialize<TKey>(ref reader);
+                reader.Read(); //propName
+                reader.Read(); //startObject
+                var value = JsonSerializer.Deserialize<TValue>(ref reader, options);
+                reader.Read(); //endKvpObject
+
+                return new KeyValuePair<TKey, TValue>(key, value);
+            }
+
+            public override void Write(Utf8JsonWriter writer, KeyValuePair<TKey, TValue> value, JsonSerializerOptions options)
+            {
+                throw new NotImplementedException();
+            }
+        }
+    }
+}