Scheduler: Terminate threads when stopping

When stopping the scheduler, particularly when destroying it, make
sure that the worker threads terminate. Otherwise deleting a running
scheduler will crash if the helper threads still access the deleted
blocks.

core/include/gnuradio-4.0/Scheduler.hpp

@@ -338,6 +338,7 @@ class SchedulerBase : public Block<Derived> {
                 this->emitErrorMessageIfAny("forEachBlock -> stop() -> LifecycleState", block.changeState(lifecycle::State::STOPPED));
             }
         });
+        waitDone();
         this->emitErrorMessageIfAny("stop() -> LifecycleState ->STOPPED", this->changeStateTo(lifecycle::State::STOPPED));
         this->emitErrorMessageIfAny("stop() -> LifecycleState ->IDLE", this->changeStateTo(lifecycle::State::IDLE));
     }