Can't access filed from xaml template through FieldInfo

I have xaml template for my MainWindow class:

...    
<Button x:Name="button1" Content="Button" Click="button1_Click"  />
<Label x:Name="superLabel" Content="Super content!" />
...

I wanna access superLabel field through relfections, after button click, like this:

public void button1_Click(object sender, RoutedEventArgs e)
{
  Type t = typeof(MainWindow);
  FieldInfo test1 = t.GetField("superLabel"); //  test1 == null
  FieldInfo test2 = t.GetRuntimeField("superLabel"); // test2 == null
  ...
}

but I am getting null in every test...