MainForm.cs

// Undo button handler
private void undoButton_Click(object sender, EventArgs e)
{
    // If history list (stack) contains any command
    if (_commands.Any())
    {
        // Pop it out and undo it
        var command = _commands.Pop();
        command.Undo();
    }
    
    // Refresh UI (trivial operation, omitted for brevity)
    RefreshWindow();
}