add null check

src/main/java/cc/fascinated/bat/features/logging/listeners/ChannelListener.java

@@ -73,6 +73,9 @@ public class ChannelListener implements EventListener {
             lastVoiceChannel.put(user, voiceChannel);
         }
         VoiceChannel voiceChannel = lastVoiceChannel.get(user);
+        if (voiceChannel == null) {
+            return;
+        }
         boolean joined = voiceChannel.getMembers().contains(event.getMember());
         if (!joined) {
             lastVoiceChannel.remove(user);