#vulnerable code 
                                @Override
    public void createClient() {
        // Create Security Object (with bootstrap server only)
        String bsUrl = "coap://" + bootstrapServer.getNonSecureAddress().getHostString() + ":"
                + bootstrapServer.getNonSecureAddress().getPort();
        Security security = new Security(bsUrl, true, 3, new byte[0], new byte[0], new byte[0], 12345);

        createClient(security);
    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Override
    public void createClient() {
        // Create Security Object (with bootstrap server only)
        String bsUrl = "coap://" + bootstrapServer.getUnsecuredAddress().getHostString() + ":"
                + bootstrapServer.getUnsecuredAddress().getPort();
        Security security = new Security(bsUrl, true, 3, new byte[0], new byte[0], new byte[0], 12345);

        createClient(security);
    }