ControlWindow

public partial class ControlWindow : Window
    {
        private double testValue = 12;

        public ControlWindow()
        {
            InitializeComponent();

            var testBox = new LabeledNumberInputBox("Время", 0, 24, (double newValue) => {
                this.testValue = newValue;
                this.Title = newValue.ToString();
            }, testValue);

            Grid.SetColumn(testBox, 0);
            controlPanel.Children.Add(testBox);
        }

        public void UpdateButtonClickHandler(object sender, RoutedEventArgs e)
        {
            // Debug.WriteLine("Update requested");

            var mainWindow = new MainWindow();
            mainWindow.DataHandler(this.testValue);
        }
    }