#vulnerable code 
                                @Override
    public void openSession(String identifier, String address, int port, long clientID) {
        PlayerCreationEvent ev = new PlayerCreationEvent(this, Player.class, Player.class, null, address, port);
        this.server.getPluginManager().callEvent(ev);
        Class<? extends Player> clazz = ev.getPlayerClass();

        try {
            Constructor constructor = clazz.getConstructor(SourceInterface.class, Long.class, String.class, Integer.class);
            Player player = (Player) constructor.newInstance(this, ev.getClientId(), ev.getAddress(), ev.getPort());
            this.players.put(identifier, player);
            this.identifiersACK.put(identifier, 0);
            this.identifiers.put(player, identifier);
            this.server.addPlayer(identifier, player);
        } catch (NoSuchMethodException | InvocationTargetException | InstantiationException | IllegalAccessException e) {
            e.printStackTrace();
        }
    } 
                            #location 3 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Override
    public void openSession(String identifier, String address, int port, long clientID) {
        PlayerCreationEvent ev = new PlayerCreationEvent(this, Player.class, Player.class, null, address, port);
        this.server.getPluginManager().callEvent(ev);
        Class<? extends Player> clazz = ev.getPlayerClass();

        try {
            Constructor constructor = clazz.getConstructor(SourceInterface.class, Long.class, String.class, int.class);
            Player player = (Player) constructor.newInstance(this, ev.getClientId(), ev.getAddress(), ev.getPort());
            this.players.put(identifier, player);
            this.identifiersACK.put(identifier, 0);
            this.identifiers.put(player, identifier);
            this.server.addPlayer(identifier, player);
        } catch (NoSuchMethodException | InvocationTargetException | InstantiationException | IllegalAccessException e) {
            e.printStackTrace();
        }
    }