Schema generation fails when a KnownType attribute specifies a method name instead of a type

[csdahlberg]

If a type uses a KnownType attribute to specify the name of a method that returns known types, a NullReferenceException is thrown when JsonSchemaGenerator.GenerateKnownTypesAsync calls JsonObjectTypeDescription.FromType. A simple example:

    public class Pencil
    {
    }

    [KnownType("GetKnownTypes")]
    public class WritingInstrument
    {
        public static Type[] GetKnownTypes()
        {
            return new[] { typeof(Pencil) };
        }
    }

    public class Container
    {
        public WritingInstrument WritingInstrument { get; set; }
    }

    public async Task ReproAsync()
    {
        var schema = await JsonSchema4.FromTypeAsync<Container>();
    }

I have support for this scenario coded, and plan on submitting a pull request shortly.

[RicoSuter]

I didnt know that this is possible. Thanks for the PR.