src/de/steamwar/bungeecore/commands/ArenaCommand.java

@@ -30,6 +30,7 @@ import net.md_5.bungee.api.config.ServerInfo;
 import net.md_5.bungee.api.connection.ProxiedPlayer;
 
 import java.util.Collection;
+import java.util.List;
 import java.util.stream.Collectors;
 
 public class ArenaCommand extends SWCommand {
@@ -62,10 +63,10 @@ public class ArenaCommand extends SWCommand {
 
             @Override
             public Collection<String> tabCompletes(CommandSender sender, String[] previousArguments, String s) {
-                return ProxyServer.getInstance().getServersCopy().values().stream().filter(server -> {
-                    Subserver subserver = Subserver.getSubserver(server);
-                    return subserver != null && subserver.getType() == Servertype.ARENA;
-                }).map(ServerInfo::getName).collect(Collectors.toList());
+                List<Subserver> subserverList = Subserver.getServerList();
+                synchronized (subserverList) {
+                    return subserverList.stream().filter(subserver -> subserver.getType() == Servertype.ARENA).map(subserver -> subserver.getServer().getName()).collect(Collectors.toList());
+                }
             }
         };
     }