#vulnerable code 
                                @Test
    public void testLayout() throws Exception {
        final Map<String, Appender> appenders = this.rootLogger.getAppenders();
        for (final Appender appender : appenders.values()) {
            this.rootLogger.removeAppender(appender);
        }
        // set up appender
        final XmlLayout layout = XmlLayout.createLayout("true", "true", "true", null, null);
        final ListAppender appender = new ListAppender("List", null, layout, true, false);
        appender.start();

        // set appender on root and set level to debug
        this.rootLogger.addAppender(appender);
        this.rootLogger.setLevel(Level.DEBUG);

        // output starting message
        this.rootLogger.debug("starting mdc pattern test");

        this.rootLogger.debug("empty mdc");

        ThreadContext.put("key1", "value1");
        ThreadContext.put("key2", "value2");

        this.rootLogger.debug("filled mdc");

        ThreadContext.remove("key1");
        ThreadContext.remove("key2");

        this.rootLogger.error("finished mdc pattern test", new NullPointerException("test"));

        final Marker marker = MarkerManager.getMarker("EVENT");
        this.rootLogger.error(marker, "marker test");

        appender.stop();

        final List<String> list = appender.getMessages();

        final String string = list.get(0);
        assertTrue("Incorrect header: " + string, string.equals("<?xml version=\"1.0\" encoding=\"UTF-8\"?>"));
        assertTrue("Incorrect footer", list.get(list.size() - 1).equals("</Events>"));
        this.checkContains("loggerFqcn=\"org.apache.logging.log4j.spi.AbstractLoggerProvider\"", list);
        this.checkContains("level=\"DEBUG\"", list);
        this.checkContains(">starting mdc pattern test</Message>", list);
        // this.checkContains("<Message>starting mdc pattern test</Message>", list);

        // <Marker xmlns="" _class="org.apache.logging.log4j.MarkerManager..Log4jMarker" name="EVENT"/>
        this.checkContains("<Marker", list);
        this.checkContains("name=\"EVENT\"/>", list);

        for (final Appender app : appenders.values()) {
            this.rootLogger.addAppender(app);
        }
    } 
                            #location 9 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void testLayout() throws Exception {
        final Map<String, Appender> appenders = this.rootLogger.getAppenders();
        for (final Appender appender : appenders.values()) {
            this.rootLogger.removeAppender(appender);
        }
        // set up appender
        final XmlLayout layout = XmlLayout.createLayout(true, true, true, false, null);
        final ListAppender appender = new ListAppender("List", null, layout, true, false);
        appender.start();

        // set appender on root and set level to debug
        this.rootLogger.addAppender(appender);
        this.rootLogger.setLevel(Level.DEBUG);

        // output starting message
        this.rootLogger.debug("starting mdc pattern test");

        this.rootLogger.debug("empty mdc");

        ThreadContext.put("key1", "value1");
        ThreadContext.put("key2", "value2");

        this.rootLogger.debug("filled mdc");

        ThreadContext.remove("key1");
        ThreadContext.remove("key2");

        this.rootLogger.error("finished mdc pattern test", new NullPointerException("test"));

        final Marker marker = MarkerManager.getMarker("EVENT");
        this.rootLogger.error(marker, "marker test");

        appender.stop();

        final List<String> list = appender.getMessages();

        final String string = list.get(0);
        assertTrue("Incorrect header: " + string, string.equals("<?xml version=\"1.0\" encoding=\"UTF-8\"?>"));
        assertTrue("Incorrect footer", list.get(list.size() - 1).equals("</Events>"));
        this.checkContains("loggerFqcn=\"org.apache.logging.log4j.spi.AbstractLoggerProvider\"", list);
        this.checkContains("level=\"DEBUG\"", list);
        this.checkContains(">starting mdc pattern test</Message>", list);
        // this.checkContains("<Message>starting mdc pattern test</Message>", list);

        // <Marker xmlns="" _class="org.apache.logging.log4j.MarkerManager..Log4jMarker" name="EVENT"/>
        this.checkContains("<Marker", list);
        this.checkContains("name=\"EVENT\"/>", list);

        for (final Appender app : appenders.values()) {
            this.rootLogger.addAppender(app);
        }
    }