How to open a page from App.xaml.cs if not open already

I have a Xaml.Forms app that uses FreshMVVM. I open a certain page from app.xaml.cs like this:

        Xamarin.Forms.Device.BeginInvokeOnMainThread(async () =>
        {
            var navService = FreshIOC.Container.Resolve<IFreshNavigationService>(FreshMvvm.Constants.DefaultNavigationServiceName);
            Page page = FreshPageModelResolver.ResolvePageModel<SomePageModel>();
            await navService.PushPage(page, null);

    ...
        });

But I need to add a check to prevent doing this if this page is already open. How can I make such a check?

Add a static bool value in the App class to check if the page has been opened:

public partial class App : Application
{
    public static bool isPageOpened;
    public App()
    {
        InitializeComponent();

        MainPage = new MainPage();
    }

    public void test()
    {

        if (App.isPageOpened = false)
        {
            Xamarin.Forms.Device.BeginInvokeOnMainThread(async () =>
            {
                var navService = FreshIOC.Container.Resolve<IFreshNavigationService>(FreshMvvm.Constants.DefaultNavigationServiceName);
                Page page = FreshPageModelResolver.ResolvePageModel<SomePageModel>();

                App.isPageOpened = true;

                await navService.PushPage(page, null);
            });
        }
    }
}

And in the page's OnDisappearing method, set the isPageOpened to false:

public partial class MainPage : ContentPage
{
    public MainPage()
    {
        InitializeComponent();
    }

    protected override void OnDisappearing()
    {
        base.OnDisappearing();

        App.isPageOpened = false;
    }
}