GUACAMOLE-2181: Avoid NPE if servlet container is shut down before extensions are loaded.

guacamole/src/main/java/org/apache/guacamole/GuacamoleServletContextListener.java

@@ -297,6 +297,7 @@ public class GuacamoleServletContextListener extends GuiceServletContextListener
             }
 
             // Inform any listeners that application shutdown has completed
+            if (listenerService != null) {
                 try {
                     listenerService.handleEvent(new ApplicationShutdownEvent() {
                         // The application shutdown event currently has no content
@@ -305,6 +306,7 @@ public class GuacamoleServletContextListener extends GuiceServletContextListener
                 catch (GuacamoleException e) {
                     logger.error("An extension listening for application shutdown failed: {}", e.getMessage(), e);
                 }
+            }
 
         }
         finally {