Probably MyTextbx and MyComboBx not been yet initialized when the ListBx_SelectionChanged is handled.
Try to check MyTextbx and MyComboBx for null
private void ListBx_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
if(MyTextbx!=null && MyComboBx!=null)
{
MyTextbx.IsEnabled = false;
MyComboBx.IsEnabled = false;
}
}