Check _cleanupTimer is not null when disposing (#4834) (#4840)

This commit checks that _cleanupTimer is not null
when disposing, which it may be if no requests have
been made.

Fixes #4818

Co-authored-by: Russ Cam <russ.cam@elastic.co>

Author: github-actions[bot]

src/Elasticsearch.Net/Connection/HandlerTracking/RequestDataHttpClientFactory.cs

@@ -154,7 +154,7 @@ protected virtual void StopCleanupTimer()
 		{
 			lock (_cleanupTimerLock)
 			{
-				_cleanupTimer.Dispose();
+				_cleanupTimer?.Dispose();
 				_cleanupTimer = null;
 			}
 		}

tests/Tests.Reproduce/GitHubIssue4818.cs

@@ -0,0 +1,22 @@
+// Licensed to Elasticsearch B.V under one or more agreements.
+// Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information
+
+using System;
+using Elastic.Elasticsearch.Xunit.XunitPlumbing;
+using FluentAssertions;
+using Nest;
+
+namespace Tests.Reproduce
+{
+	public class GitHubIssue4818
+	{
+		[U]
+		public void ConnectionSettingsDoesNotThrowNullReferenceException()
+		{
+			IDisposable settings = new ConnectionSettings();
+			Action func = () => settings.Dispose();
+			func.Should().NotThrow<NullReferenceException>();
+		}
+	}
+}