How to get to ApplicationBar from PopUp window?

I am trying to password protect my application. I selected the PopUp window in which the user can enter his password.

My application contains ApplicatioBar, so I need to hide it when the password input dialog is displayed. I do the following:

public MainPage() 
{
    ...
    PasscodeApplicationBarVisibility(); //Now AppBar disapears
    PasscodeEvent();
}

private void PasscodeApplicationBarVisibility() 
{
    ((ApplicationBar)ApplicationBar).IsVisible = !(settings.PasscodeRequired); 
}

private void PasscodeEvent() 
{
    passwordInput = new PasswordInputPrompt {
    Title = "Please Enter Passcode!",
    InputScope = new InputScope { Names = { new InputScopeName() { NameValue = InputScopeNameValue.TelephoneNumber } } }, };

    passwordInput.Completed += new EventHandler<PopUpEventArgs<string, PopUpResult>>(passwordInput_Completed);    
    passwordInput.Show();
}

void passwordInput_Completed(object sender, PopUpEventArgs<string, PopUpResult> e) 
{
    if (settings.Passcode.Equals(e.Result)) {
        settings.PasscodeRequired = false;

        //Here is the problem - NullReferenceException
        PasscodeApplicationBarVisibility();

        ((PasswordInputPrompt)sender).Completed -= passwordInput_Completed; }
    else { 
        ((PasswordInputPrompt)sender).Completed -= passwordInput_Completed; 
        IncorrectPasscode = true; 
        PasscodeEvent(); }          
}

ApplicationBar disappears as it should be. When the user enters a valid password, the application should show an ApplicationBar. But when I try to get to the ApplicationBar from the PopUp window, I get NullReferenceExceptionthat, which is understandable because the PopUp windows do not contain the ApplicationBar. I don't know how to get to MainPage ApplicationBar when the PopUp window is open.

Please any hints?

UPDATE:

, , . , , PasswordInputPrompt Coding4Fun. , PasswordInputPrompt System.Windows.Controls.Primitives.PopUp. , . , PopUp.

, Coding4Fun Parent. : passwordInput.Parent.Dispatcher.BeginInvoke(PasscodeApplicationBarVisibility);