Navigation Best Practice?

Hey all,

I am currently working on a cross platform app and need some advice as to the best practice when handling navigation pages.

Currently I have a login/sign in page where if a successful login occurs then I will store the session token to handle persistent authentication so the user won't need to enter the username and password every time they kill and reopen the app:

public App()
    {
        InitializeComponent();

        Init();
    }

private async void Init()
    {
        bool validToken = await SecureStorage.GetAsync(Constants.AUTH_TOKEN_ID) != null;
        if (validToken)
        {
            MainPage = new NavigationPage(NavigationManager.MainPage);
        }
        else
        {
            MainPage = new NavigationPage(NavigationManager.SignInPage);
        }
    }

When Login Button is clicked and successful authenticates credentials:

private async void LoginButton_Clicked(object sender, EventArgs e)
    {
        try
        {
            await Navigation.PushAsync(NavigationManager.MainPage);
        }
        catch(Exception ex)
        {
            await DisplayAlert("Error.", ex.Message, "Ok");
            return;
        }
    }

If the user decides to logout manually then I am clearing the tokens from secure storage and setting the main page to be SignInPage again:

private async void LogoutButton_Clicked(object sender, EventArgs e)
    {
        App.Current.MainPage = new NavigationPage(NavigationManager.SignInPage);
    }

My problem is that if I now try to enter user credentials and sign back I get the error message "Page must not already have a parent"

Note: NavigationManager is a singleton that is used to store instances for each page instead of constantly creating new instances.

public static class NavigationManager
{
    private static SignInPage signInPage;
    public static SignInPage SignInPage
    {
        get
        {
            if (signInPage == null)
            {
                signInPage = new SignInPage();
            }

            return signInPage;
        }
    }

    private static MainPage mainPage;
    public static MainPage MainPage
    {
        get
        {
            if(mainPage == null)
            {
                mainPage = new MainPage();
            }

            return mainPage;
        }
    }
}

Anyone have any suggestions on how to fix this error? Or a better way of handling navigation.