Navigationservice is not working in silverlight 4

Hi,
 i am using below code to navigate from one xaml page to  another page.but this  is not working . it displays "Object Reference" error
 
 

NavigationService.Navigate(new Uri("/MainPage", UriKind.Relative));
 


I am using silverlight 4 with RIA service

try it in following two ways
Page pge = new Page();
pge.NavigationService.Navigate(new Uri("URL",UriKind.Relative));
 
or
 
this.NavigationService.Navigate(new Uri("URL",UriKind.Relative));