RestSharp updating use of AddHandler mathod to use factory delegate
According to the source code at https://github.com/restsharp/RestSharp/blob/dev/src/RestSharp/RestClient.cs:
[Obsolete("Use the overload that accepts a factory delegate")]
public void AddHandler(string contentType, IDeserializer deserializer) =>
AddHandler(contentType, () => deserializer);
The obsolete overload just calls the AddHandler(string contentType, Func<IDeserializer> deserializerFactory)
overload.
So you can replace your code to add your custom handler as follows:
RestClient.AddHandler("application/json", () => { return CustomJsonSerializer.Instance; });