Navigation Bar appears for few seconds and disappears

I am navigating between Login form and MainPage of application using following code:

Navigation.PushAsync (App.GetMainPage ());

One the MainPage I am trying to remove the NavigationBar using following lines of code:

protected override void OnAppearing()
    {
        base.OnAppearing();
        NavigationPage.SetHasNavigationBar (this, false);
        NavigationPage.SetHasBackButton (this, false);
        ViewModel.RefreshData();
    }

The problem is that NavigationBar appears for few second and disappears. Also the back button remains in the navigation bar.

Any suggestions on how to resolve both of these issues?