fix OWEwoksWidgetOneThreadPerRun

When processing several time the OWEwoksWidgetOneThreadPerRun widget we end up with the following error:

    def __disconnect_all_task_executors(self):
        for task_executor, _ in self.__task_executors.values():
>           task_executor.finished.disconnect(self._ewoks_task_finished_callback)
E           TypeError: 'method' object is not connected