Make Set concurrent

SpigotCore_Main/src/de/steamwar/entity/REntityServer.java

@@ -33,6 +33,7 @@ import org.bukkit.event.Listener;
 import org.bukkit.event.player.PlayerMoveEvent;
 import org.bukkit.event.player.PlayerQuitEvent;
 
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Set;
@@ -69,7 +70,7 @@ public class REntityServer implements Listener {
     private final HashMap<Player, Integer> viewDistance = new HashMap<>();
 
     private EntityActionListener callback = null;
-    private final Set<Player> playersThatClicked = new HashSet<>();
+    private final Set<Player> playersThatClicked = Collections.synchronizedSet(new HashSet<>());
 
     private final BiFunction<Player, Object, Object> filter = (player, packet) -> {
         REntity entity = entityMap.get(useEntityTarget.get(packet));