Fix

SpigotCore_Main/src/de/steamwar/inventory/SWListInv.java

@@ -28,6 +28,7 @@ import org.bukkit.entity.Player;
 import org.bukkit.event.inventory.ClickType;
 
 import java.util.*;
+import java.util.stream.Collectors;
 
 public class SWListInv<T> extends SWInventory {
 
@@ -55,7 +56,7 @@ public class SWListInv<T> extends SWInventory {
     public void open(){
         opened = true;
         inventory.clear();
-        callbacks.keySet().stream().filter(i -> i < 0).forEach(callbacks::remove);
+        callbacks.keySet().stream().filter(i -> i >= 0).collect(Collectors.toList()).forEach(callbacks::remove);
         if(!callbacks.containsKey(-999)) {
             setCallback(-999, (ClickType click) -> player.closeInventory());
         }