Master Detail navigation issues in Xamarin Forms

I'm having problems understanding how Master Detail navigation works in Xamarin Forms, let's go to my code as it is now:

1- I created a interface for navigation:

using System.Threading.Tasks;

namespace MobileApp.Services
{
    public interface INavigationService
    {
        Task NavigateToLogin();
        Task NavigateToRegister();
        Task NavigateToMain();
        Task NavigateToCoupons();
        Task NavigateToProfile();
    }
}

2 - I created the classe that implements the interface :

using MobileApp.Views;
using System.Threading.Tasks;
using System;

namespace MobileAPP.Services
{
    public class NavigationService : INavigationService
    {
        public async Task NavigateToCoupons()
        {
            await App.Current.MainPage.Navigation.PushAsync(new CouponsView());
        }

        public async Task NavigateToLogin()
        {
            await App.Current.MainPage.Navigation.PushAsync(new LoginView());
        }

        public async Task NavigateToMain()
        {
            await App.Current.MainPage.Navigation.PushAsync(new MainView());
        }

        public async Task NavigateToRegister()
        {
            await App.Current.MainPage.Navigation.PushAsync(new RegisterView());
        }

        public async Task NavigateToProfile()
        {
            await App.Current.MainPage.Navigation.PushAsync(new ProfileView());
        }
    }
}

3- This is my MainView that inherits from MasterDetailPage and sets it to master and detail .

using Xamarin.Forms;

namespace MobileApp.Views
{
    public partial class MainView : MasterDetailPage
    {
        public MainView()
        {
            InitializeComponent();

            this.Master = new MasterView();
            this.Detail = new NavigationPage(new DetailView());
        }
    }
}

4- My home screen is view of Login , and I only have método where I check my credentials. Login ok, I already use interface of navigation to MainView and it works perfectly, at this point I have no problem. Follow código :

private async void Login()
        {
            var IsAuthenticated = await _authenticate.Login(Email, Password);
            if (!string.IsNullOrEmpty(IsAuthenticated))
            {
                await this._navigationService.NavigateToMain();
            }
            else
            {
                await this._messageService.ShowAsync("Email ou senha invalidos");
            }
        }

5- The problem occurs in MasterView , which is the side menu. In it I have several Button that lead to other pages, it follows código of MasterViewModel :

using MobileApp.Services;
using System.Windows.Input;
using Xamarin.Forms;
using System;

namespace MobileApp.ViewModels
{
    class MasterViewModel : ViewModelBase
    {
        private readonly IMessageService _messageService;
        private readonly INavigationService _navigationService;

        public ICommand CouponsCommand { get; set; }
        public ICommand ProfileCommand { get; set; }

        public MasterViewModel()
        {
            this.CouponsCommand = new Command(this.Coupons);
            this.ProfileCommand = new Command(this.Profile);
        }

        private async void Profile()
        {
            await this._navigationService.NavigateToProfile();
        }

        private async void Coupons()
        {
            await this._navigationService.NavigateToCoupons();
        }
    }
}

When I try to navigate, the error occurs: Object reference not set to an instance of an object. when clicking on any button. But as you can see in the implementation of interface of navigation I'm instanciando a página , just like it happens after Login which leads to MainView and works fine. If I remove the código that uses the interface _navigationService and put the same código that is in classe NavigationService in Profile , for example:

private async void Profile()
{
   await App.Current.MainPage.Navigation.PushAsync(new ProfileView());
}

private async void Coupons()
{
   await this._navigationService.NavigateToCoupons();
}

So the navigation works, I really did not understand what's going wrong. An observation; I'm not using any framework to MVVM (type mvvmcross, prismmvvm, etc) because first I want to learn how it all works. Can anyone explain where the error is?