public void removeNullElements()
    {
        FastSet<L2Character>        knownCharactersTemp = new FastSet<L2Character>();
        for (L2Character element: knownCharacters)
            if (element != null) knownCharactersTemp.add(element);
        knownCharacters.clear();
        knownCharacters.addAll(knownCharactersTemp);

        FastSet<L2MonsterInstance>  knownMonstersTemp   = new FastSet<L2MonsterInstance>();
        for (L2MonsterInstance element: knownMonsters)
            if (element != null) knownMonstersTemp.add(element);
        knownMonsters.clear();
        knownMonsters.addAll(knownMonstersTemp);

        FastSet<L2NpcInstance>      knownNpcsTemp       = new FastSet<L2NpcInstance>();
        for (L2NpcInstance element: knownNpcs)
            if (element != null) knownNpcsTemp.add(element);
        knownNpcs.clear();
        knownNpcs.addAll(knownNpcsTemp);

        FastSet<L2Object>           knownObjectsTemp    = new FastSet<L2Object>();
        for (L2Object element: knownObjects)
            if (element != null) knownObjectsTemp.add(element);
        knownObjects.clear();
        knownObjects.addAll(knownObjectsTemp);

        FastSet<L2PcInstance>       knownPlayersTemp    = new FastSet<L2PcInstance>();
        for (L2PcInstance element: knownPlayers)
            if (element != null) knownPlayersTemp.add(element);
        knownPlayers.clear();
        knownPlayers.addAll(knownPlayersTemp);
    }