#vulnerable code 
                                public void startTlsClient(TlsConfig config) {
        TlsContext tlsContext = new TlsContext(config);
        WorkflowExecutor workflowExecutor = WorkflowExecutorFactory.createWorkflowExecutor(config.getExecutorType(),
                tlsContext);

        try {
            workflowExecutor.executeWorkflow();
        } catch (WorkflowExecutionException ex) {
            LOGGER.info("The TLS protocol flow was not executed completely, follow the debug messages for more information.");
            LOGGER.debug(ex.getLocalizedMessage(), ex);
        }

        if (config.getWorkflowOutput() != null && !config.getWorkflowOutput().isEmpty()) {
            FileOutputStream fos = null;
            try {
                fos = new FileOutputStream(config.getWorkflowOutput());
                WorkflowTraceSerializer.write(fos, tlsContext.getWorkflowTrace());
            } catch (FileNotFoundException ex) {
                java.util.logging.Logger.getLogger(TLSClient.class.getName()).log(Level.SEVERE, null, ex);
            } catch (JAXBException | IOException ex) {
                LOGGER.info("Could not serialize WorkflowTrace.");
                LOGGER.debug(ex);
            } finally {
                try {
                    fos.close();
                } catch (IOException ex) {
                    LOGGER.debug(ex);
                }
            }
        }
    } 
                            #location 25 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public void startTlsClient(TlsConfig config) {
        TlsContext tlsContext = new TlsContext(config);
        WorkflowExecutor workflowExecutor = WorkflowExecutorFactory.createWorkflowExecutor(config.getExecutorType(),
                tlsContext);

        try {
            workflowExecutor.executeWorkflow();
        } catch (WorkflowExecutionException ex) {
            LOGGER.info("The TLS protocol flow was not executed completely, follow the debug messages for more information.");
            LOGGER.debug(ex.getLocalizedMessage(), ex);
        }
    }