bugfixes, better background operation robustness and platform-specific error handling

lib/model/account_data.dart

@@ -2,7 +2,6 @@ import 'dart:async';
 import 'dart:convert';
 
 import 'package:crypto/crypto.dart';
-import 'package:firebase_messaging/firebase_messaging.dart';
 import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
@@ -57,14 +56,6 @@ class AccountData {
       .convert(utf8.encode('${getUsername()}:${getPassword()}'))
       .toString();
 
-  Future<String> getDeviceId() async => sha512
-      .convert(
-        utf8.encode(
-          '${getUserSecret()}@${await FirebaseMessaging.instance.getToken()}',
-        ),
-      )
-      .toString();
-
   Future<void> setData(String username, String password) async {
     await _secureStorage.write(key: _usernameField, value: username);
     await _secureStorage.write(key: _passwordField, value: password);