#vulnerable code 
                                @Test
    public void getRequestIdGets() throws IOException
    {
        //arrange
        String validString = "$iothub/twin/res/?$rid=5";
        TopicParser testParser = new TopicParser(validString);

        //act
        String status = testParser.getRequestId(3);

        //assert
        assertNotNull(status);
        assertTrue(status.equals(String.valueOf(5)));
    } 
                            #location 13 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void getRequestIdGets() throws IOException
    {
        //arrange
        String validString = "$iothub/twin/res/?$rid=5";
        TopicParser testParser = new TopicParser(validString);

        //act
        String status = Deencapsulation.invoke(testParser, "getRequestId", 3);

        //assert
        assertNotNull(status);
        assertTrue(status.equals(String.valueOf(5)));
    }