❔ Need help with Event Handlers to update a field.

System.NullReferenceException
System.Windows.Controls.Primitives.Selector.SelectedItem.get returned null.

        private void InputWaarde_TextChanged(object sender, TextChangedEventArgs e)
        {
            OutputWaarde.Text = Formules.Lengte(InputEenheid.SelectedItem.ToString(), OutputEenheid.SelectedItem.ToString(), InputWaarde.Text);

        }
        
        private void InputEenheid_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            OutputWaarde.Text = Formules.Lengte(InputEenheid.SelectedItem.ToString(), OutputEenheid.SelectedItem.ToString(), InputWaarde.Text);

        }

        private void OutputEenheid_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            OutputWaarde.Text = Formules.Lengte(InputEenheid.SelectedItem.ToString(), OutputEenheid.SelectedItem.ToString(), InputWaarde.Text);

        }