try to reproduce #3538 to no avail

Author: Mpdreamz

src/Tests/Tests.Reproduce/GithubIssue3538.cs

@@ -0,0 +1,95 @@
+using System;
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using System.Threading;
+using Elastic.Xunit.XunitPlumbing;
+using FluentAssertions;
+using Nest;
+using Nest.JsonNetSerializer;
+using Newtonsoft.Json;
+using Tests.Core.Client.Settings;
+
+namespace Tests.Reproduce
+{
+	public class GithubIssue3538
+	{
+		[U] public void EmptyPolicyCausesNullReferenceException()
+		{
+			var converter = new MyPocoJsonConverter();
+			var connectionSettings = new AlwaysInMemoryConnectionSettings(
+					sourceSerializerFactory: (builtin, settings) => new JsonNetSerializer(builtin, settings,
+						() =>
+						{
+							var jSettings = new JsonSerializerSettings();
+							jSettings.Converters.Add(converter);
+							return jSettings;
+						}))
+				.DefaultIndex("blah");
+
+			var client = new ElasticClient(connectionSettings);
+
+			var result = client.IndexDocument(new MyPoco() { Id = 1, Name = "x" });
+			converter.CanConvertCallCount.Should().Be(1);
+
+			client.Bulk(b => b.Index<MyPoco>(i=>i.Document(new MyPoco { Id = 2, Name = "y" })));
+			converter.CanConvertCallCount.Should().Be(2);
+
+			client.Bulk(b => b.Update<MyPoco>(i=>i.Doc(new MyPoco { Id = 3, Name = "z" })));
+			converter.CanConvertCallCount.Should().Be(3);
+
+			client.Bulk(b => b.Update<MyPoco>(i=>i.Upsert(new MyPoco { Id = 4, Name = "a" })));
+			converter.CanConvertCallCount.Should().Be(4);
+
+			converter.WriteCount.Should().Be(4);
+			converter.SeenIds.Should().BeEquivalentTo(1, 2, 3, 4);
+
+		}
+
+		public class MyPoco
+		{
+			public int Id { get; set; }
+			public string Name { get; set; }
+		}
+
+		public class MyPocoJsonConverter : JsonConverter
+		{
+			private int _canConvertCallCount;
+			public int CanConvertCallCount => _canConvertCallCount;
+
+			private int _writeCount;
+			public int WriteCount => _writeCount;
+
+			public List<int> SeenIds { get; } = new List<int>();
+
+
+			public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+			{
+				Interlocked.Increment(ref _writeCount);
+
+				if (!(value is MyPoco poco))
+				{
+					writer.WriteNull();
+					return;
+				}
+
+				writer.WriteStartObject();
+				writer.WritePropertyName("id");
+				writer.WriteValue(poco.Id);
+				SeenIds.Add(poco.Id);
+				writer.WritePropertyName("name");
+				writer.WriteValue(poco.Name);
+				writer.WriteEndObject();
+			}
+
+			public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) =>
+				new MyPoco { };
+
+			public override bool CanConvert(Type objectType)
+			{
+				Interlocked.Increment(ref _canConvertCallCount);
+
+				return objectType == typeof(MyPoco);
+			}
+		}
+	}
+}