https://issues.apache.org/jira/browse/AMQ-6291

Simplify the selector close in doStop, the run method no longer tries to
close this after it has been registered and assigned.

activemq-client/src/main/java/org/apache/activemq/transport/tcp/TcpTransportServer.java

@@ -501,10 +501,9 @@ public class TcpTransportServer extends TransportServerThreadSupport implements
         Exception firstFailure = null;
 
         try {
-            final Selector selector = this.selector;
             if (selector != null) {
-                this.selector = null;
                 selector.close();
+                selector = null;
             }
         } catch (Exception error) {
         }