public void Login(Credential _credential, Action<bool, string> _callback)
    {
        auth.SignInWithCredentialAsync(_credential).ContinueWithOnMainThread(_result =>
        {
            if (_result.Exception != null)
            {
                _callback?.Invoke(false, "Failed to login: " + _result.Exception.Message);
            }
            else
            {
                firebaseUser = _result.Result;
                _callback?.Invoke(true, string.Empty);
            }
        });
    }