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

@@ -74,7 +74,11 @@ public class SWInventory implements Listener {
 
     public void setItem(int pos, ItemStack itemStack, InvCallback c) {
         inventory.setItem(pos, itemStack);
-        callbacks.put(pos, inventoryClickEvent -> c.clicked(inventoryClickEvent.getClick()));
+        if(c != null) {
+            callbacks.put(pos, inventoryClickEvent -> c.clicked(inventoryClickEvent.getClick()));
+        } else {
+            callbacks.remove(pos);
+        }
     }
 
     public void setItem(int pos, SWItem item) {