public ModalWindowViewModel()
        {
            ShowTextCommand.Subscribe(_ =>
            {
                NumberOutput.Value = NumberInput.Value;
                NumberInput.Value  = null;

                FocusTextBoxCommand.Execute();
            });

            ClearTextCommand = NumberOutput
                               .Select(number => !(number is null))
                               .ToReactiveCommand();
            ClearTextCommand.Subscribe(_ =>
            {
                NumberOutput.Value = null;
                NumberInput.Value  = null;

                FocusTextBoxCommand.Execute();
            });
        }