Add isNullAction() method to fix compiler warning

apps/openmw/mwgui/sortfilteritemmodel.cpp

@@ -132,9 +132,12 @@ namespace MWGui
                 && !base.get<ESM::Book>()->mBase->mData.mIsScroll)
             return false;
 
-        if ((mFilter & Filter_OnlyUsableItems) && typeid(*base.getClass().use(base)) == typeid(MWWorld::NullAction)
-                && base.getClass().getScript(base).empty())
-            return false;
+        if ((mFilter & Filter_OnlyUsableItems) && base.getClass().getScript(base).empty())
+        {
+            boost::shared_ptr<MWWorld::Action> actionOnUse = base.getClass().use(base);
+            if (!actionOnUse || actionOnUse->isNullAction())
+                return false;
+        }
 
         return true;
     }

apps/openmw/mwworld/action.hpp

@@ -32,6 +32,9 @@ namespace MWWorld
 
             virtual ~Action();
 
+            virtual bool isNullAction() { return false; }
+            ///< Is running this action a no-op? (default false)
+
             void execute (const Ptr& actor);
 
             void setSound (const std::string& id);

apps/openmw/mwworld/nullaction.hpp

@@ -9,6 +9,8 @@ namespace MWWorld
     class NullAction : public Action
     {
             virtual void executeImp (const Ptr& actor) {}
+
+            virtual bool isNullAction() { return true; }
     };
 }