Fix null reference issue for invalid query.

backend/src/Squidex.Infrastructure/Json/Newtonsoft/JsonClassConverter.cs

@@ -28,7 +28,7 @@ namespace Squidex.Infrastructure.Json.Newtonsoft
             return ReadValue(reader, objectType, serializer);
         }
 
-        protected abstract T ReadValue(JsonReader reader, Type objectType, JsonSerializer serializer);
+        protected abstract T? ReadValue(JsonReader reader, Type objectType, JsonSerializer serializer);
 
         public sealed override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
         {

backend/src/Squidex.Infrastructure/Json/Newtonsoft/SurrogateConverter.cs

@@ -12,11 +12,11 @@ namespace Squidex.Infrastructure.Json.Newtonsoft
 {
     public sealed class SurrogateConverter<T, TSurrogate> : JsonClassConverter<T> where T : class where TSurrogate : ISurrogate<T>, new()
     {
-        protected override T ReadValue(JsonReader reader, Type objectType, JsonSerializer serializer)
+        protected override T? ReadValue(JsonReader reader, Type objectType, JsonSerializer serializer)
         {
             var surrogate = serializer.Deserialize<TSurrogate>(reader);
 
-            return surrogate!.ToSource();
+            return surrogate?.ToSource();
         }
 
         protected override void WriteValue(JsonWriter writer, T value, JsonSerializer serializer)