app crashes when trying to send multiple fcm messages programmatically - xamarin.forms

Question

i am trying to create a messaging app using xamarin.forms. i created a send button and added the following code:
private async void send_Clicked(object sender, EventArgs e)
{
await Task.Run(() => SendNotification(token, "title", message.Text));
}
and the sendnotification is as follows:
public string SendNotification(string DeviceToken, string title, string msg)
{
var result = "-1";
var httpWebRequest = (HttpWebRequest)WebRequest.Create(webAddr);
httpWebRequest.ContentType = "application/json";
httpWebRequest.Headers.Add(string.Format("Authorization: key={0}", serverKey));
httpWebRequest.Headers.Add(string.Format("Sender: id={0}", senderId));
httpWebRequest.Method = "POST";
var payload = new
{
//to= "/topics/klm",
to = DeviceToken,
//to = "egjLx6VdFS0:APA91bGQMSSRq_wCzywNC01zJi4FBtHXrXuL-p4vlkl3a3esdH8lxo7mQZUBlrTi7h-6JXx0GrJbwc9Vx6M5Q4OV_3CArcdlP0XMBybervQvfraWvqCgaa75gu9SVzjY4V_qd36JGg4A",
priority = "high",
content_available = true,
data = new
{
text = msg,
},
};
var serializer = new JsonSerializer();
using (var streamWriter = new StreamWriter(httpWebRequest.GetRequestStream()))
{
string json = JsonConvert.SerializeObject(payload);
streamWriter.Write(json);
streamWriter.Flush();
}
var httpResponse = (HttpWebResponse)httpWebRequest.GetResponse();
using (var streamReader = new StreamReader(httpResponse.GetResponseStream()))
{
result = streamReader.ReadToEnd();
}
return result;
}
this is private string webAddr = "https://fcm.googleapis.com/fcm/send";
when i click send the first time, i receive the message perfectly, when i hit send the second time, the app freezes then crashes and asks me if i want to close the app or wait. why is this happening? thanks in advance