dataflow_cpp: fixed bug with global network instances in mtapi scheduler

dataflow_cpp/include/embb/dataflow/internal/scheduler_mtapi.h

@@ -66,15 +66,18 @@ class SchedulerMTAPI : public Scheduler {
     }
     }
   }
   }
   virtual ~SchedulerMTAPI() {
   virtual ~SchedulerMTAPI() {
+    if (embb::tasks::Node::IsInitialized()) {
+      // only destroy groups and queues if there still is an instance
       embb::tasks::Node & node = embb::tasks::Node::GetInstance();
       embb::tasks::Node & node = embb::tasks::Node::GetInstance();
       for (int ii = 0; ii < slices_; ii++) {
       for (int ii = 0; ii < slices_; ii++) {
         group_[ii]->WaitAll(MTAPI_INFINITE);
         group_[ii]->WaitAll(MTAPI_INFINITE);
         node.DestroyGroup(*group_[ii]);
         node.DestroyGroup(*group_[ii]);
       }
       }
-    embb::base::Allocation::Free(group_);
       for (int ii = 0; ii < queue_count_; ii++) {
       for (int ii = 0; ii < queue_count_; ii++) {
         node.DestroyQueue(*queue_[ii]);
         node.DestroyQueue(*queue_[ii]);
       }
       }
+    }
+    embb::base::Allocation::Free(group_);
     embb::base::Allocation::Free(queue_);
     embb::base::Allocation::Free(queue_);
   }
   }
   virtual void Spawn(Action & action) {
   virtual void Spawn(Action & action) {