Async await not wait the process using Visual Stud

Question

i don't know why my async not waiting the process, here is my code :

userKu = new UserData();
btnLogin = FindViewById<Button>(Resource.Id.buttonLogin);

protected override void OnCreate(Bundle bundle)
{
    base.OnCreate(bundle);
    SetContentView(Resource.Layout.Main);
    if (isOnline() == true)
    {
        AppPreferences ap = new AppPreferences(mContext);
        string usernameCekLogin = ap.getNamaUser(); 

        //TOAST #1
        Toast.MakeText(this, userKu.status, ToastLength.Long).Show();
        isLogin(usernameCekLogin, userKu);

        //TOAST #2
        Toast.MakeText(this, userKu.status, ToastLength.Long).Show();

        btnLogin.Click +=  delegate{}
    }
}

private async Task<string> FetchUserAsync(string url)
{
    // Create an HTTP web request using the URL:
    HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(new Uri(url));
    request.ContentType = "application/json";
    request.Method = "GET";

    // Send the request to the server and wait for the response:
    using (WebResponse response = await request.GetResponseAsync())
    {
        // Get a stream representation of the HTTP web response:
        using (var sr = new StreamReader(response.GetResponseStream()))
        {
            string strContent = sr.ReadToEnd();
            return strContent;
        }
    }
}

private async void isLogin(string username, UserData data1)
{
    string url = "http://localhost.com/api.php/isLogin/login_mbr/"+username;

    data1 = JsonConvert.DeserializeObject<UserData>(await FetchUserAsync(url));

    //TOAST 3
    Toast.MakeText(this, "check user status = "+data1.status, ToastLength.Long).Show();
}

When I run it, I don't get the value on #TOAST 1 and #TOAST 2, but on #TOAST 3 I get the value "0" or "1".

If I change protected override async void OnCreate(Bundle bundle) and I don't use function like this :

userKu = new UserData();
btnLogin = FindViewById<Button>(Resource.Id.buttonLogin);

protected override async void OnCreate(Bundle bundle)
{
    base.OnCreate(bundle);
    SetContentView(Resource.Layout.Main);
    if (isOnline() == true)
    {
        string url = "http://localhost.com/api.php/isLogin/login_mbr/";
        AppPreferences ap = new AppPreferences(mContext);

        string usernameCekLogin = ap.getNamaUser(); 
        url += usernameCekLogin;
        //TOAST 1
        Toast.MakeText(this, userKu.status, ToastLength.Long).Show();
        userKu = JsonConvert.DeserializeObject<UserData>(await FetchUserAsync(url));
        //TOAST 2
        Toast.MakeText(this, userKu.status, ToastLength.Long).Show();
        /*
            btnLogin.Click +=  delegate{}*/
    }
}

private async Task<string> FetchUserAsync(string url)
{
    // Create an HTTP web request using the URL:
    HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(new Uri(url));
    request.ContentType = "application/json";
    request.Method = "GET";

    // Send the request to the server and wait for the response:
    using (WebResponse response = await request.GetResponseAsync())
    {
        // Get a stream representation of the HTTP web response:
        using (var sr = new StreamReader(response.GetResponseStream()))
        {
            string strContent = sr.ReadToEnd();
            return strContent;
        }
    }
}

On #Toast 1 I don't get value and on #Toast 2 I get the value. But I get error when I enable my button, it said null references. Any Solution for my problems? I'm using Visual Studio 2015 and Xamarin for developing android app.

Thanks in advance