private async void Entry_Unfocused_Username(object sender, FocusEventArgs e)
{
Entry username = (Entry)FindByName("username");
bool result = await model.CheckUsername(username.Text);
if (result)
{
username.TextColor = Color.Red;
await Application.Current.MainPage.DisplayAlert("Greška", "Korisničko ime je zauzeto !", "OK");
}
else
{
username.TextColor = Color.Black;
}
}