Regression in 1.2: NullReferenceException when serializing null record field

[cmeeren]

Repro:

open System
open System.Text.Json
open System.Text.Json.Serialization

type X = { X: X }

let x = { X = Unchecked.defaultof<_> }

let options = JsonSerializerOptions()
options.Converters.Add(JsonFSharpOptions.Default() |> JsonFSharpConverter)

Console.WriteLine(JsonSerializer.Serialize(x, options))

On v1.1 this works fine, but on v1.2 this throws:

System.NullReferenceException: Object reference not set to an instance of an object.
   at lambda_method2(Closure, Object)
   at System.Text.Json.Serialization.JsonRecordConverter`1.WriteRestOfObject(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
   at System.Text.Json.Serialization.JsonRecordConverter`1.Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
   at System.Text.Json.Serialization.JsonConverter`1.TryWrite(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.Serialization.JsonConverter`1.WriteCore(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.Serialization.JsonConverter`1.WriteCoreAsObject(Utf8JsonWriter writer, Object value, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.JsonSerializer.WriteCoreAsObject(Utf8JsonWriter writer, Object value, JsonTypeInfo jsonTypeInfo)
   at System.Text.Json.Serialization.JsonRecordConverter`1.WriteRestOfObject(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
   at System.Text.Json.Serialization.JsonRecordConverter`1.Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
   at System.Text.Json.Serialization.JsonConverter`1.TryWrite(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.Serialization.JsonConverter`1.WriteCore(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.JsonSerializer.WriteCore[TValue](Utf8JsonWriter writer, TValue& value, JsonTypeInfo`1 jsonTypeInfo)
   at System.Text.Json.JsonSerializer.WriteString[TValue](TValue& value, JsonTypeInfo`1 jsonTypeInfo)
   at System.Text.Json.JsonSerializer.Serialize[TValue](TValue value, JsonSerializerOptions options)
   at <StartupCode$Test2>.$Program.main@() in \_\Test2\Test2\Program.fs:line 12

[cmeeren]

I have identified the commit this bug was introduced as 8ab76c9. This only enabled the flag HandleNull in Record.fs, so it's something about the code invoked when using that flag.

[Tarmil]

Fixed in v1.2.42, thanks for the PR!

[cmeeren]

Excellent, thanks a lot!