Fix NPE

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

@@ -103,7 +103,7 @@ public class SWInventory implements Listener {
     }
 
     public void setCallback(int pos, InvCallback c) {
-        callbacks.put(pos, inventoryClickEvent -> c.clicked(inventoryClickEvent.getClick()));
+        callbacks.put(pos, inventoryClickEvent -> c.clicked(inventoryClickEvent == null ? null : inventoryClickEvent.getClick()));
     }
 
     public void setItemEvent(int pos, ItemStack itemStack, Consumer<InventoryClickEvent> c) {