diff --git a/platform/core.execution/src/org/netbeans/core/execution/DefaultSysProcess.java b/platform/core.execution/src/org/netbeans/core/execution/DefaultSysProcess.java
index 6d2b6c3d4509..d11e169bdd02 100644
--- a/platform/core.execution/src/org/netbeans/core/execution/DefaultSysProcess.java
+++ b/platform/core.execution/src/org/netbeans/core/execution/DefaultSysProcess.java
@@ -101,35 +101,4 @@ public void run() {
     public String getName() {
         return name;
     }
-    
-    /** destroy the thread group this process was handled from. Not that simple
-     * as it seems, since the ThreadGroup can't be destroyed from inside.
-     */
-    void destroyThreadGroup(ThreadGroup base) {
-        new Thread(base, new Destroyer(group)).start();
-    }
-    private static class Destroyer implements Runnable {
-        private final ThreadGroup group;
-        Destroyer(ThreadGroup group) {
-            this.group = group;
-        }
-        @Override public void run() {
-            try {
-                while (group.activeCount() > 0) {
-                    Thread.sleep(1000);
-                }
-            }
-            catch (InterruptedException e) {
-                Exceptions.printStackTrace(e);
-            }
-            if (!group.isDestroyed()) {
-                try {
-                    group.destroy();
-                } catch (IllegalThreadStateException x) {
-                    // #165302: destroyed some other way?
-                }
-            }
-        }
-    }
-
 }
diff --git a/platform/core.execution/src/org/netbeans/core/execution/ExecutionEngine.java b/platform/core.execution/src/org/netbeans/core/execution/ExecutionEngine.java
index 3ae04281f636..f554209df38e 100644
--- a/platform/core.execution/src/org/netbeans/core/execution/ExecutionEngine.java
+++ b/platform/core.execution/src/org/netbeans/core/execution/ExecutionEngine.java
@@ -71,9 +71,9 @@
 
     /* table of window:threadgrp */
     private static final WindowTable wtable = new WindowTable();
-
+    
     /** list of ExecutionListeners */
-    private final HashSet<ExecutionListener> executionListeners = new HashSet<>();
+    private final Set<ExecutionListener> executionListeners;
 
     /** List of running executions */
     private final List<ExecutorTask> runningTasks = Collections.synchronizedList(new ArrayList<>(5));
@@ -87,6 +87,8 @@
     static final long serialVersionUID =9072488605180080803L;
 
     public ExecutionEngine () {
+        this.executionListeners = new HashSet<>();
+        
         /* SysIn is a class that redirects System.in of some running task to
            a window (probably OutWindow).
            SysOut/Err are classes that redirect out/err to the window
@@ -134,7 +136,6 @@ public String getRunningTaskName( ExecutorTask task ) {
     @Override
     public ExecutorTask execute(String name, Runnable run, final InputOutput inout) {
         TaskThreadGroup g = new TaskThreadGroup(base, "exec_" + name + "_" + number); // NOI18N
-        g.setDaemon(true);
         ExecutorTaskImpl task = new ExecutorTaskImpl();
         synchronized (task.lock) {
             try {
@@ -192,9 +193,12 @@ protected final PermissionCollection createPermissions(CodeSource cs, InputOutpu
 
     /** fires event that notifies about new process */
     protected final void fireExecutionStarted (ExecutionEvent ev) {
-        runningTasks.add( ev.getProcess() );
-	@SuppressWarnings("unchecked") 
-        Iterator<ExecutionListener> iter = ((HashSet<ExecutionListener>) executionListeners.clone()).iterator();
+        runningTasks.add(ev.getProcess());
+
+        Iterator<ExecutionListener> iter;
+        synchronized (executionListeners) {
+            iter = (new HashSet<>(executionListeners)).iterator();
+        }
         while (iter.hasNext()) {
             ExecutionListener l = iter.next();
             l.startedExecution(ev);
@@ -202,15 +206,17 @@ protected final void fireExecutionStarted (ExecutionEvent ev) {
     }
 
     /** fires event that notifies about the end of a process */
-    protected final void fireExecutionFinished (ExecutionEvent ev) {
-        runningTasks.remove( ev.getProcess() );
-	@SuppressWarnings("unchecked") 
-        Iterator<ExecutionListener> iter = ((HashSet<ExecutionListener>) executionListeners.clone()).iterator();
+    protected final void fireExecutionFinished(ExecutionEvent ev) {
+        runningTasks.remove(ev.getProcess());
+
+        Iterator<ExecutionListener> iter;
+        synchronized (executionListeners) {
+            iter = (new HashSet<>(executionListeners)).iterator();
+        }
         while (iter.hasNext()) {
             ExecutionListener l = iter.next();
             l.finishedExecution(ev);
         }
-        ev.getProcess().destroyThreadGroup(base);
     }
 
     static void putWindow(java.awt.Window w, TaskThreadGroup tg) {
@@ -296,5 +302,4 @@ public void close() throws IOException {
     static PrintStream createPrintStream(boolean stdOut) {
         return new WriterPrintStream(new SysOut(stdOut), stdOut);
     }
-   
 }
diff --git a/platform/openide.util/src/org/openide/util/RequestProcessor.java b/platform/openide.util/src/org/openide/util/RequestProcessor.java
index eb91719c57bb..d1f8e2ee400e 100644
--- a/platform/openide.util/src/org/openide/util/RequestProcessor.java
+++ b/platform/openide.util/src/org/openide/util/RequestProcessor.java
@@ -1892,20 +1892,14 @@ static Processor get() {
                             return proc;
                         }
                     } else {
-                        assert checkAccess(TOP_GROUP.getTopLevelThreadGroup());
                         Processor proc = POOL.pop();
                         proc.idle = false;
-
                         return proc;
                     }
                 }
                 newP = new Processor();
             }
         }
-        private static boolean checkAccess(ThreadGroup g) throws SecurityException {
-            g.checkAccess();
-            return true;
-        }
 
         /** A way of returning a Processor to the inactive pool.
          *