public bool IsOpen {get;}

{
  // find the first project task item which is open
  var taskItem = Project.Current.GetItems<TaskProjectItem>().FirstOrDefault(t => t.IsOpen == true);
  // if there isn't a project task item, return
  if (taskItem == null)
    return;

  if (taskItem.IsOpen)
  {
    // close it
    // NOTE : The task item will also be removed from the project
    //At 2.x -
    //TaskAssistantModule.CloseTaskAsync(taskItem.TaskItemGuid);
    TaskAssistantFactory.Instance?.CloseTaskItemAsync(taskItem.TaskItemGuid);
  }
}