Fix a null reference case in session context

Author: fredericDelaporte

src/NHibernate.Test/NHSpecificTest/Logs/LogsFixture.cs

@@ -192,6 +192,17 @@ public void WillGetSessionIdFromSessionLogsConcurrent()
 			}
 		}
 
+		[Test]
+		public void WillCleanlyFailOnDoubleProcessDispose()
+		{
+			using (var s = OpenSession())
+			{
+				var p = ((AbstractSessionImpl) s).BeginProcess();
+				p.Dispose();
+				Assert.That(() => p.Dispose(), Throws.TypeOf<ObjectDisposedException>());
+			}
+		}
+
 		// IFixingRequired interface ensures the value is evaluated at log time rather than at log buffer flush time.
 		public class SessionIdCapturer : IFixingRequired
 		{

src/NHibernate/Impl/AbstractSessionImpl.cs

@@ -329,7 +329,7 @@ public ProcessHelper(AbstractSessionImpl session)
 
 			public void Dispose()
 			{
-				_context.Dispose();
+				_context?.Dispose();
 				_context = null;
 				if (_session == null)
 					throw new ObjectDisposedException("The session process helper has been disposed already");