Increasing thread safety

src/de/steamwar/bungeecore/Subserver.java

@@ -151,7 +151,9 @@ public class Subserver implements Runnable {
             fatalError();
             Thread.currentThread().interrupt();
         } finally {
+            synchronized (serverList){
                 serverList.remove(this);
+            }
             ProxyServer.getInstance().getServers().remove(serverName);
             shutdownCallback.run();
         }