A simple ListPicker with 2 string options, as shown in the samples, when the event handler is added to a page and this page is navigated to, the event Fires before object initialization, causing NullReferenceException over that exact listPicker object.
private void listTheme_SelectionChanged(object sender, SelectionChangedEventArgs e)
if (listTheme.SelectedItem != null)
The exception is thrown when navigating to the page containing the listPicker, showing that "listTheme" is in fact 'null'.
Sender isn't null but doesn't have a SelectedItem.