[219500] Added a guard for a disposed session to avoid a RejectedExecutionException.

plugins/org.eclipse.dd.dsf.debug/src/org/eclipse/dd/dsf/debug/service/StepQueueManager.java

@@ -237,12 +237,14 @@ public class StepQueueManager extends AbstractDsfService
                     new DsfRunnable() { public void run() {
                         fTimedOutFutures.remove(e.getDMContext());
 
+                        if (getSession().isActive()) {
                             // Issue the stepping time-out event.
                             getSession().dispatchEvent(
                                 new ISteppingTimedOutEvent() { 
                                     public IExecutionDMContext getDMContext() { return e.getDMContext(); }
                                 }, 
                                 getProperties());
+                        }
                     }},
                     STEPPING_TIMEOUT, TimeUnit.MILLISECONDS)
                 );