How to fix System.NullReferenceException error?

Question

WPF

There are 2 wpf components on the form

ComboBox Name =  "ShowThisMessageToComboBox"   SelectionChanged = "ShowThisMessageToComboBox_Changed"
TextBox Name = "ProgrammIdsTextBox"

It is necessary to hide the textbox when selecting item 0 in the combobox, and show it when selecting 1.

Here is the code

private void ShowThisMessageToComboBox_Changed(object sender, SelectionChangedEventArgs e)
        {
            if (ShowThisMessageToComboBox.SelectedIndex == 0) ProgrammIdsTextBox.Visibility = Visibility.Hidden; // Ошибка на этой строке
            if (ShowThisMessageToComboBox.SelectedIndex == 1) ProgrammIdsTextBox.Visibility = Visibility.Visible;
        }

As a result, I get the error System.NullReferenceException: 'Object reference not set to an instance of an object.'
Most likely there is no reference to the object.

PS
The similar logic worked in WinForms.

Answer 1

Check if you have a TextBox in a container? Then his program will not see.