docs: public <T> Future<T> addTask(String title,
                                   long total,
                                   ProgressAsyncHandler<T> handler) {
          if (executor.isShutdown()) {
              throw new IllegalStateException("Adding task to closed progress manager");
          }
  
          InternalTask<T> task = new InternalTask<>(title, total, handler);
          queuedTasks.add(task);
          startTasks();
          return task;
      }