Hotfix NPE

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

@@ -92,7 +92,7 @@ public class SWInventory implements Listener {
             return;
 
         e.setCancelled(true);
-        if(callbacks.containsKey(e.getRawSlot()))
+        if(callbacks.containsKey(e.getRawSlot()) && callbacks.get(e.getRawSlot()) != null)
             callbacks.get(e.getRawSlot()).clicked(e.getClick());
     }