Unit Testing for Page navigation from viewmodel is not working

hi all
I want write unit test for navigate to Mastedetail Page from my LoginViewModel
My case is:
Application.Current.MainPage.Navigation.PushModalAsync(new MainPage());
here my MainPage is Masterdetai Page and here if write Unit Test case like this

    [Test]
    public async Task Test1()
    {
        var lvm = new LoginViewModel();
        lvm.Username = "admin";
        lvm.Password = "admin";
        var app = new App();

        var page = new ContentPage();

        Application.Current.MainPage = new MainPage();
        await Application.Current.MainPage.Navigation.PushModalAsync(page1);
        string result = lvm.LoginResult();
        Assert.IsTrue(result == "Success", "test fail");
        Assert.Pass();
    }

its giving System.InvalidOperationException: You MUST call Xamarin.Forms.Init(); prior to using it, error at master detail page line MasterBehavior = MasterBehavior.Popover;

if any one have idea please let me know how to get Application.current.mainpage in unit test project and how to resolve this issue
Thanks In Advance