How to stop Dispatch timer from another window in WPF

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WPF

i want to stop timer from another window but i am getting exception as
Object reference not set to an instance of an object.
My code is
MainWindow.xaml

public  DispatcherTimer timer = new DispatcherTimer();

       public MainWindow()
       {
           InitializeComponent();

           timer.Interval = TimeSpan.FromSeconds(1);
           timer.Tick += timer_Tick;
           timer.Start();

       }

       void timer_Tick(object sender, EventArgs e)
       {
           lblTime.Content = DateTime.Now.ToLongTimeString();
       }

       private void Button_Click(object sender, RoutedEventArgs e)
       {
           Window1 obj = new Window1();
           obj.ShowDialog();
       }

Window1.xaml

public Window1()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            MainWindow obj = new MainWindow();
            obj = Application.Current.Windows.OfType<MainWindow>().SingleOrDefault(x => x.IsActive);
            obj.timer.Stop();
        }