Check for inactive workers and remove their references.

def _clean(self):
        """Check for inactive workers and remove their references."""
        if self._workers:
            for w in self._workers:
                if w.is_finished():
                    self._workers.remove(w)

        if self._threads:
            for t in self._threads:
                if t.isFinished():
                    self._threads.remove(t)
        else:
            self._timer.stop()