linter and lint fixes

flutter/lib/models/user.dart

@@ -41,24 +41,24 @@ class User {
 
   factory User.fromJson(Map<String, dynamic> json) {
     return User(
-      userID: json['user_id'],
-      username: json['username'],
-      timestampCreated: json['timestamp_created'],
-      timestampLastRead: json['timestamp_lastread'],
-      timestampLastSent: json['timestamp_lastsent'],
-      messagesSent: json['messages_sent'],
-      quotaUsed: json['quota_used'],
-      quotaRemaining: json['quota_remaining'],
-      quotaPerDay: json['quota_max'],
-      isPro: json['is_pro'],
-      defaultChannel: json['default_channel'],
-      maxBodySize: json['max_body_size'],
-      maxTitleLength: json['max_title_length'],
-      defaultPriority: json['default_priority'],
-      maxChannelNameLength: json['max_channel_name_length'],
-      maxChannelDescriptionLength: json['max_channel_description_length'],
-      maxSenderNameLength: json['max_sender_name_length'],
-      maxUserMessageIDLength: json['max_user_message_id_length'],
+      userID: json['user_id'] as String,
+      username: json['username'] as String?,
+      timestampCreated: json['timestamp_created'] as String,
+      timestampLastRead: json['timestamp_lastread'] as String?,
+      timestampLastSent: json['timestamp_lastsent'] as String?,
+      messagesSent: json['messages_sent'] as int,
+      quotaUsed: json['quota_used'] as int,
+      quotaRemaining: json['quota_remaining'] as int,
+      quotaPerDay: json['quota_max'] as int,
+      isPro: json['is_pro'] as bool,
+      defaultChannel: json['default_channel'] as String,
+      maxBodySize: json['max_body_size'] as int,
+      maxTitleLength: json['max_title_length'] as int,
+      defaultPriority: json['default_priority'] as int,
+      maxChannelNameLength: json['max_channel_name_length'] as int,
+      maxChannelDescriptionLength: json['max_channel_description_length'] as int,
+      maxSenderNameLength: json['max_sender_name_length'] as int,
+      maxUserMessageIDLength: json['max_user_message_id_length'] as int,
     );
   }
 }