check if the guild is valid before getting it

src/main/java/cc/fascinated/bat/service/GuildService.java

@@ -59,6 +59,9 @@ public class GuildService extends ListenerAdapter {
         if (guilds.containsKey(id)) {
             return guilds.get(id);
         }
+        if (DiscordService.JDA.getGuildById(id) == null) {
+            return null;
+        }
         long start = System.currentTimeMillis();
         Optional<BatGuild> optionalGuild = guildRepository.findById(id);