如何停止调度程序计时器？

Question

我试图在不使用计时器时停止它，并在需要时在以后的计时器上启动它。如果我尝试使用timer.Stop()停止计时器，则会显示“对象引用未设置为对象的实例。”。如果我尝试在初始化函数之外启动计时器，即使我的timer是全局的，我也会收到相同的错误消息。

如何在Initialise函数之外控制计时器，但调用相同的方法？ C＃：

public class SmartDispatcherTimer : DispatcherTimer
{
    public SmartDispatcherTimer()
    {

        base.Tick += SmartDispatcherTimer_Tick;
    }

    async void SmartDispatcherTimer_Tick(object sender, EventArgs e)
    {
        if (TickTask == null)
        {
            Console.WriteLine("No task set!");
            return;
        }

        if (IsRunning && !IsReentrant)
        {
            // previous task hasn't completed
            Console.WriteLine("Task already running");
            return;
        }

        try
        {
            // we're running it now
            IsRunning = true;
            Console.WriteLine("Running Task");

            await TickTask.Invoke();
            Console.WriteLine("Task Completed");

        }
        catch (Exception ex)
        {
            Console.WriteLine("Task Failed");
        }
        finally
        {
            // allow it to run again
            IsRunning = false;
        }
    }

    public bool IsReentrant { get; set; }
    public bool IsRunning { get; private set; }

    public Func<Task> TickTask { get; set; }
}

private SmartDispatcherTimer timer;

private void Initialise()
{
    try
    {
        timer = new SmartDispatcherTimer()
        {
            IsReentrant = true,
            Interval = TimeSpan.FromSeconds(1),
            TickTask = async () => { await GetData(); }
        };

        timer.Start();

    }
    catch (Exception ex)
    {
        MessageBox.Show("Task Failed");
    }
}