a nullable exception occure using Frame.NavigateTo

Question

i am getting a nullable exception when try to navigate in page with class object data using Frame.NavigateTo in visual studio 2013 window store application with c#/xaml

my some code part here  

 

 

 

private void Button_Click_3(object sender, RoutedEventArgs e)

{

adress adr = new adress();

ComboBoxItem cnt = (ComboBoxItem)combobox_cnt.SelectedItem;

adr.country = cnt.Content.ToString();

ComboBoxItem st = (ComboBoxItem)combobox_st.SelectedItem;

adr.state = st.Content.ToString();

citydata ct = (citydata)combobox_ct.SelectedItem;

adr.city = ct.city.ToString();

//this.Content = new ResultPage1();

this.Frame.Navigate(typeof(ResultPage1),adr);

}

 

Frame is initialize to null at the time of debugging