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);
}