System.NullReferenceException Handled

Question

Here's code I have written for a simple event based multiplier , this is the error I constantly get: Object reference not set to an instance of an object ath this line: txt3.Text = num3.ToString();.
I am a beginner, so what's my error here? What am I missing? Thanks. All the TextBoxes have value set in XAML.

public partial class MainWindow : Window

    {

        public MainWindow()

        {

            InitializeComponent();

        }

        private void txt2_TextChanged(object sender, TextChangedEventArgs e)

        {

            int num1, num2, num3;

            num1 = int.Parse(txt1.Text);

            num2 = int.Parse(txt2.Text);

            num3 = num1 * num2;

            txt3.Text = num3.ToString();

        }

    }