Set the query string, but still get NullReferenceException - windows-phone-7

Question

I want to show the error message on the other page. I got the NullReferenceException, but the query string is set on the page which has error. Would someone tell me what is wrong with my code?
catch (Exception ex)
{
//Dispatcher.BeginInvoke(new Action(() =>MessageBox.Show(ex.StackTrace,"Error!",MessageBoxButton.OK)));
string query=#"/ErrorPage.xaml?msg=" + ex.StackTrace.ToString() ;
Dispatcher.BeginInvoke(new Action(() =>this.NavigationService.Navigate(new Uri(query, UriKind.Relative))));
}
There is the code for showing the error message when the page is loaded on other page
public ErrorPage()
{
InitializeComponent();
string msg = NavigationContext.QueryString["msg"].ToString();
lstMessage.Items.Add(msg);
}

Answer 1

I should put my code into the MainPage_Load method. It works.
public ErrorPage()
{
InitializeComponent();
this.Loaded += new RoutedEventHandler(MainPage_Loaded);
}
private void MainPage_Loaded(object sender, RoutedEventArgs e)
{
MessageBoxResult result = MessageBox.Show(CMSPhoneApp.App.GlobalVariables.strNofifyEmailSubject,
"Report Error", MessageBoxButton.OKCancel);
if (result == MessageBoxResult.OK)
{
//according to the serach it works on real devices (not on the emulator)
//the reason the EmailComposer not pop up because can't set up an email account on the emulator
EmailComposeTask emailcomposer = new EmailComposeTask();
emailcomposer.To = CMSPhoneApp.App.GlobalVariables.reportAddress;
emailcomposer.Subject = CMSPhoneApp.App.GlobalVariables.strNofifyEmailSubject;
emailcomposer.Body = CMSPhoneApp.App.GlobalVariables.errorMsg;
emailcomposer.Show();
}
else
{
App.GoBack();
}
}