Fix exception in ConcreteTypeConverter when _score is null

Closes #1863
Closes #1929

Author: gmarz

Diff for: src/Nest/CommonAbstractions/SerializationBehavior/StatefulDeserialization/ConcreteTypeConverter.cs

@@ -66,14 +66,14 @@ public override object ReadJson(JsonReader reader, Type objectType, object exist
 			instance.Fields = new FieldValues(serializer.GetConnectionSettings().Inferrer, instance.Fields);
 			return instance;
 		}
-	
+
 		public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
 		{
 			throw new NotSupportedException();
 		}
 	}
 
-	internal static class ConcreteTypeConverter 
+	internal static class ConcreteTypeConverter
 	{
 		internal static object GetUsingConcreteTypeConverter<T>(
 			JsonReader reader, JsonSerializer serializer, ConcreteTypeConverter<T> realConcreteConverter)
@@ -103,8 +103,8 @@ private static object GetHitTypeInstance(Type concreteType)
 		}
 
 		internal static Type GetConcreteTypeUsingSelector<T>(
-			JsonSerializer serializer, 
-			ConcreteTypeConverter<T> realConcreteConverter, 
+			JsonSerializer serializer,
+			ConcreteTypeConverter<T> realConcreteConverter,
 			JObject jObject)
 			where T: class
 		{
@@ -122,7 +122,7 @@ internal static Type GetConcreteTypeUsingSelector<T>(
 			hitDynamic.Fields = fieldValues;
 			hitDynamic.Source = d._source;
 			hitDynamic.Index = d._index;
-			hitDynamic.Score = d._score;
+			hitDynamic.Score = d._score != null ? (double)d._score : 0;
 			hitDynamic.Type = d._type;
 			hitDynamic.Version = d._version;
 			hitDynamic.Id = d._id;
@@ -136,4 +136,4 @@ internal static Type GetConcreteTypeUsingSelector<T>(
 			return concreteType;
 		}
 	}
-}
+}

Diff for: src/Nest/Search/Search/Hits/Hit.cs

@@ -37,11 +37,12 @@ public class Hit<T> : IHit<T>
 
 		[JsonProperty(PropertyName = "_index")]
 		public string Index { get; internal set; }
-		
+
 		[JsonProperty(PropertyName = "inner_hits")]
 		[JsonConverter(typeof(VerbatimDictionaryKeysJsonConverter))]
 		public IDictionary<string, InnerHitsResult> InnerHits { get; internal set; }
-		
+
+		// TODO make this nullable for 5.0
 		[JsonProperty(PropertyName = "_score")]
 		public double Score { get; set; }
 

Diff for: src/Tests/Reproduce/GithubIssue1863.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Elasticsearch.Net;
+using Nest;
+using FluentAssertions;
+using Tests.Framework;
+using Xunit;
+using Tests.Framework.Integration;
+using Tests.Framework.MockData;
+
+namespace Tests.Reproduce
+{
+	[Collection(IntegrationContext.ReadOnly)]
+	public class GithubIssue1863
+	{
+		[I]
+		public void ConcreteTypeConverterThrowsExceptionOnNullScore()
+		{
+			var client = TestClient.GetClient();
+			var response = client.Search<Project>(s => s
+				.ConcreteTypeSelector((d,h) => typeof(Project))
+				.Sort(srt => srt.Ascending(p => p.StartedOn))
+			);
+			response.Hits.Count().Should().BeGreaterThan(0);
+			response.Hits.All(h => h.Score == 0).Should().BeTrue();
+		}
+	}
+}