Fix ArgumentNullException in MultiSearch OIS when SearchRequest is untyped

Closes #1279

Author: gmarz

src/Nest/Resolvers/Converters/MultiSearchConverter.cs

@@ -96,7 +96,7 @@ public override object ReadJson(JsonReader reader, Type objectType, object exist
 			{
 				var descriptor = m.Descriptor.Value;
 				var concreteTypeSelector = descriptor.TypeSelector;
-				var baseType = m.Descriptor.Value.ClrType;
+				var baseType = m.Descriptor.Value.ClrType ?? typeof(object);
 
 				var generic = MakeDelegateMethodInfo.MakeGenericMethod(baseType);
 

src/Tests/Nest.Tests.Integration/Nest.Tests.Integration.csproj

@@ -178,6 +178,7 @@
     <Compile Include="Reproduce\Reproduce994Tests.cs" />
     <Compile Include="Reproduce\Reproduce986Tests.cs" />
     <Compile Include="Reproduce\Reproduce960Tests.cs" />
+    <Compile Include="Reproduce\Reproduce1279Tests.cs" />
     <Compile Include="Search\Filter\AndOrNotFilterTests.cs" />
     <Compile Include="Search\Filter\ScriptFilterTests.cs" />
     <Compile Include="Search\Filter\MissingExistsFilterTests.cs" />

src/Tests/Nest.Tests.Integration/Reproduce/Reproduce1279Tests.cs

@@ -0,0 +1,40 @@
+using NUnit.Framework;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using FluentAssertions;
+using Nest.Tests.MockData.Domain;
+using Newtonsoft.Json.Linq;
+
+namespace Nest.Tests.Integration.Reproduce
+{
+	[TestFixture]
+	public class Reproduce1279Tests : IntegrationTests
+	{
+		[Test]
+		public void MultiSearchNullArgumentException()
+		{
+			var request = new MultiSearchRequest
+			{
+				Operations = new Dictionary<string, ISearchRequest>
+				{
+					{ "test", new SearchRequest
+						{
+							Query = new QueryContainer(new MatchAllQuery()),
+							Types = new TypeNameMarker[] { typeof(Product), typeof(ElasticsearchProject) },
+							TypeSelector = (o, h) => typeof(ElasticsearchProject)
+						}
+					}
+				}
+			};
+
+			var result = Client.MultiSearch(request);
+			result.IsValid.Should().BeTrue();
+			var response = result.GetResponse<object>("test");
+			var projects = response.Documents.OfType<ElasticsearchProject>().ToList();
+			projects.Count.Should().BeGreaterThan(0);
+		}
+	}
+}