Check for Authentication on evry view-Xamarin Forms lab

I have an application with few views. I want to be able to check if user is logged in when any view appears. I am using Xamarin Forms Labs to implement solution in an Mvvm way. To implement this functionality, I created a baseviewmodel and in its constructor I check for logged in status. If the user is not logged in then I want to pop open the login view. Here is the code I used-

public class AuthenticateBaseViewModel:Xamarin.Forms.Labs.Mvvm.ViewModel,IAuthentication
{
public AuthenticateBaseViewModel ()
{
CheckAuthentication ();
}

    #region IAuthentication implementation
    public void CheckAuthentication ()
    {
        if(!App.IsLoggedIn)
            Navigation.PushModalAsync<LoginViewModel>();
    }
    #endregion


}

However Navigation is null and hence I keep getting null reference exception. Even though the BaseViewModel is inheriting from Labs Mvvm class, why is the Navigation null?

Thanks

Apurva