我想我已经找到解决问题的办法了。
我们通过使用. NET的Firebase库创建连接对象来调用Firebase。有时，示例化为FirebaseApp的对象被初始化为null，我理解该对象一次只应该存在一个示例，函数的异步性质不能很好地处理这个问题。
我们选择通过对Firebase API进行REST调用来创建通知，并解决了问题

GoogleCredential gc = GoogleCredential.FromJson(googleJson);
string tok = await gc.CreateScoped("https://www.googleapis.com/auth/firebase.messaging https://www.googleapis.com/auth/cloud-platform").UnderlyingCredential.GetAccessTokenForRequestAsync();

HttpClient httpCli = new HttpClient() { BaseAddress = new Uri("https://fcm.googleapis.com/v1/projects/xxxxxx-57b13/messages:send"), };
httpCli.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", tok);
var content = new StringContent(m, Encoding.UTF8, "application/json");

using HttpResponseMessage response = await httpCli.PostAsync("", content);