#fixed code
                             @Test
	public void shouldCreateConversationThenJoinAndSendOfferRequest() throws Exception {
		// given
		MessageMatcher s1Matcher = new MessageMatcher();
		MessageMatcher s2Matcher = new MessageMatcher();
		Session s1 = mockSession("s1", s1Matcher);
		Session s2 = mockSession("s2", s2Matcher);
		server.register(s1);
		server.register(s2);

		// when
		server.handle(Message.create()//
				.signal("create")//
				.build(), s1);
		String conversationKey = s1Matcher.getMessage().getContent();
		s1Matcher.reset();
		server.handle(Message.create()//
				.signal("join")//
				.content(conversationKey)//
				.build(), s2);

		// then
		assertThat(s1Matcher.getMessages().size(), is(2));
		assertMatch(s1Matcher, 0, "s2", "s1", "joined", EMPTY);
		assertMatch(s1Matcher, 1, "s2", "s1", "offerRequest", EMPTY);

		assertThat(s2Matcher.getMessages().size(), is(1));
		assertMatch(s2Matcher, 0, EMPTY, "s2", "joined", conversationKey);
	}
#vulnerable code 
                             @Test
	public void shouldCreateConversationThenJoinAndSendOfferRequest() throws Exception {
		// given
		MessageMatcher s1Matcher = new MessageMatcher();
		MessageMatcher s2Matcher = new MessageMatcher();
		Session s1 = mockSession("s1", s1Matcher);
		Session s2 = mockSession("s2", s2Matcher);
		server.register(s1);
		server.register(s2);

		// when
		server.handle(Message.create()//
				.signal("create")//
				.build(), s1);
		String conversationKey = s1Matcher.getMessage().getContent();
		s1Matcher.reset();
		server.handle(Message.create()//
				.signal("join")//
				.content(conversationKey)//
				.build(), s2);

		// then
		assertThat(s1Matcher.getMessages().size(), is(2));
		assertThat(s1Matcher.getMessage().getFrom(), is("s2"));
		assertThat(s1Matcher.getMessage().getTo(), is("s1"));
		assertThat(s1Matcher.getMessage().getSignal(), is("joined"));

		assertThat(s1Matcher.getMessage(1).getFrom(), is("s2"));
		assertThat(s1Matcher.getMessage(1).getTo(), is("s1"));
		assertThat(s1Matcher.getMessage(1).getSignal(), is("offerRequest"));

		assertThat(s2Matcher.getMessages().size(), is(1));
		assertThat(s2Matcher.getMessage().getSignal(), is("joined"));
		assertThat(s2Matcher.getMessage().getContent(), is(conversationKey));
	} 
                            #location 30 
                            #vulnerability type NULL_DEREFERENCE