#vulnerable code 
                                public void createPSKClient(String pskIdentity, byte[] pskKey) {

        // Create Security Object (with bootstrap server only)
        String bsUrl = "coaps://" + bootstrapServer.getSecureAddress().getHostString() + ":"
                + bootstrapServer.getSecureAddress().getPort();
        byte[] pskId = pskIdentity.getBytes(StandardCharsets.UTF_8);
        Security security = Security.pskBootstrap(bsUrl, pskId, pskKey);

        createClient(security);
    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public void createPSKClient(String pskIdentity, byte[] pskKey) {

        // Create Security Object (with bootstrap server only)
        String bsUrl = "coaps://" + bootstrapServer.getSecuredAddress().getHostString() + ":"
                + bootstrapServer.getSecuredAddress().getPort();
        byte[] pskId = pskIdentity.getBytes(StandardCharsets.UTF_8);
        Security security = Security.pskBootstrap(bsUrl, pskId, pskKey);

        createClient(security);
    }