如何检查身份验证C＃Firebase

Question

我对object reference not set to an instance of an object in this code有疑问：

 private async void ButtonLogin_Click(object sender, EventArgs e)
        {
            try
            {
                FirebaseResponse firebaseResponse = await client.GetTaskAsync("User/" + textBoxUsername.Text);
                DataLogin datar = firebaseResponse.ResultAs<DataLogin>();

                if (textBoxPassword.Text == datar.password)
                {
                    MessageBox.Show("Success");
                }

            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }