Make npc on transport world object. Try to fix the bug that npc are unloaded with grid and disappear.

src/server/game/Entities/Transport/Transport.cpp

@@ -673,6 +673,7 @@ uint32 Transport::AddNPCPassenger(uint32 tguid, uint32 entry, float x, float y,
         return 0;
     }
 
+    creature->m_isWorldObject = true; //so it will not be unloaded with grid
     map->AddToMap(creature);
     m_NPCPassengerSet.insert(creature);