Ghost presence fixes

Author: Hackzzila

lib/src/events/PresenceUpdateEvent.dart

@@ -14,8 +14,9 @@ class PresenceUpdateEvent {
       if (data['user'].length > 1) {
         data['user'] = data['user'] as Map<String, dynamic>;
       } else {
-        data['user'] = client.users[data['user']['id']]._raw;
+        data['user'] = client.users[data['user']['id']]?._raw;
       }
+      if (data['user'] == null) return;
 
       this.newMember = new Member._new(client, data);
       this.oldMember = newMember.guild.members[newMember.id];