#vulnerable code 
                                @Override
    public void backchannelLogout(UserSessionModel userSession, ClientSessionModel clientSession) {
        ClientModel client = clientSession.getClient();
        if (!(client instanceof ApplicationModel)) return;
        ApplicationModel app = (ApplicationModel)client;
        if (app.getManagementUrl() == null) return;

        SAML2LogoutRequestBuilder logoutBuilder = new SAML2LogoutRequestBuilder()
                                         .userPrincipal(userSession.getUser().getUsername())
                                         .destination(client.getClientId());
        if (requiresRealmSignature(client)) {
            logoutBuilder.signatureAlgorithm(getSignatureAlgorithm(client));
            logoutBuilder.sign(realm.getPrivateKey(), realm.getPublicKey());
        }
        /*
        if (requiresEncryption(client)) {
            PublicKey publicKey = null;
            try {
                publicKey = PemUtils.decodePublicKey(client.getAttribute(ClientModel.PUBLIC_KEY));
            } catch (Exception e) {
                logger.error("failed", e);
                return;
            }
            logoutBuilder.encrypt(publicKey);
        }
        */

        String logoutRequestString = null;
        try {
            logoutRequestString = logoutBuilder.postBinding().encoded();
        } catch (Exception e) {
            logger.warn("failed to send saml logout", e);
            return;
        }


        String adminUrl = ResourceAdminManager.getManagementUrl(uriInfo.getRequestUri(), app);

        ApacheHttpClient4Executor executor = ResourceAdminManager.createExecutor();


        try {
            ClientRequest request = executor.createRequest(adminUrl);
            request.formParameter(GeneralConstants.SAML_REQUEST_KEY, logoutRequestString);
            request.formParameter(SAML2LogOutHandler.BACK_CHANNEL_LOGOUT, SAML2LogOutHandler.BACK_CHANNEL_LOGOUT);
            ClientResponse response = null;
            try {
                response = request.post();
                response.releaseConnection();
                // Undertow will redirect root urls not ending in "/" to root url + "/".  Test for this weird behavior
                if (response.getStatus() == 302  && !adminUrl.endsWith("/")) {
                    String redirect = (String)response.getHeaders().getFirst(HttpHeaders.LOCATION);
                    String withSlash = adminUrl + "/";
                    if (withSlash.equals(redirect)) {
                        request = executor.createRequest(withSlash);
                        request.formParameter(GeneralConstants.SAML_REQUEST_KEY, logoutRequestString);
                        request.formParameter(SAML2LogOutHandler.BACK_CHANNEL_LOGOUT, SAML2LogOutHandler.BACK_CHANNEL_LOGOUT);
                        response = request.post();
                        response.releaseConnection();
                    }
                }
            } catch (Exception e) {
                logger.warn("failed to send saml logout", e);
            }

        } finally {
            executor.getHttpClient().getConnectionManager().shutdown();
        }

    } 
                            #location 51 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Override
    public void backchannelLogout(UserSessionModel userSession, ClientSessionModel clientSession) {
        ClientModel client = clientSession.getClient();
        if (!(client instanceof ApplicationModel)) return;
        ApplicationModel app = (ApplicationModel)client;
        if (app.getManagementUrl() == null) return;

        SAML2LogoutRequestBuilder logoutBuilder = new SAML2LogoutRequestBuilder()
                                         .userPrincipal(userSession.getUser().getUsername())
                                         .destination(client.getClientId());
        if (requiresRealmSignature(client)) {
            logoutBuilder.signatureAlgorithm(getSignatureAlgorithm(client))
                         .signWith(realm.getPrivateKey(), realm.getPublicKey())
                         .signDocument();
        }
        /*
        if (requiresEncryption(client)) {
            PublicKey publicKey = null;
            try {
                publicKey = PemUtils.decodePublicKey(client.getAttribute(ClientModel.PUBLIC_KEY));
            } catch (Exception e) {
                logger.error("failed", e);
                return;
            }
            logoutBuilder.encrypt(publicKey);
        }
        */

        String logoutRequestString = null;
        try {
            logoutRequestString = logoutBuilder.postBinding().encoded();
        } catch (Exception e) {
            logger.warn("failed to send saml logout", e);
            return;
        }


        String adminUrl = ResourceAdminManager.getManagementUrl(uriInfo.getRequestUri(), app);

        ApacheHttpClient4Executor executor = ResourceAdminManager.createExecutor();


        try {
            ClientRequest request = executor.createRequest(adminUrl);
            request.formParameter(GeneralConstants.SAML_REQUEST_KEY, logoutRequestString);
            request.formParameter(SAML2LogOutHandler.BACK_CHANNEL_LOGOUT, SAML2LogOutHandler.BACK_CHANNEL_LOGOUT);
            ClientResponse response = null;
            try {
                response = request.post();
                response.releaseConnection();
                // Undertow will redirect root urls not ending in "/" to root url + "/".  Test for this weird behavior
                if (response.getStatus() == 302  && !adminUrl.endsWith("/")) {
                    String redirect = (String)response.getHeaders().getFirst(HttpHeaders.LOCATION);
                    String withSlash = adminUrl + "/";
                    if (withSlash.equals(redirect)) {
                        request = executor.createRequest(withSlash);
                        request.formParameter(GeneralConstants.SAML_REQUEST_KEY, logoutRequestString);
                        request.formParameter(SAML2LogOutHandler.BACK_CHANNEL_LOGOUT, SAML2LogOutHandler.BACK_CHANNEL_LOGOUT);
                        response = request.post();
                        response.releaseConnection();
                    }
                }
            } catch (Exception e) {
                logger.warn("failed to send saml logout", e);
            }

        } finally {
            executor.getHttpClient().getConnectionManager().shutdown();
        }

    }