When a polymorphic interface type used for an object, deserialization throwing NRE

[buyaa-n]

I am aware of ploymorphism not supported yet, but i think JsonException should be thrown instead of NRE

        public class MyClass
        {
            public string Value { get; set; }

            public IThing Thing { get; set; }
        }

        public interface IThing
        {
            int Number { get; set; }
        }

        public class MyThing : IThing
        {
            public int Number { get; set; }
        }

        [Fact]
        public void AssertDoesDeserialize()
        {
            const string json = @"{
""Value"": ""A value"",
""Thing"": {
""Number"": 123
}
}";
            Assert.Throws<JsonException>(() =>
            {
                JsonSerializer.Parse<List<MyClass>>(json);
            });
            /* this test works if not use any interface type, but i guess that is expected as polymorphism not supported yet
                        MyClass myClass = JsonSerializer.Parse<MyClass>(json);
                        Assert.NotNull(myClass);
                        Assert.Equal("A value", myClass.Value);
                        Assert.NotNull(myClass.Thing);
                        Assert.Equal(123, myClass.Thing.Number);
            */
        }

[ahsonkhan]

This is one of the specific instances related to https://github.com/dotnet/corefx/issues/37545 and should be fixed. cc @steveharter

[MarcoRossignoli]

Up for grab?I'm working also on other similar