#vulnerable code 
                             @Test
	public void writeXml_null() {
		DateTimePropertyImpl prop = new DateTimePropertyImpl(null);

		Document actual = xcalProperty(marshaller);
		marshaller.writeXml(prop, XmlUtils.getRootElement(actual));

		Document expected = xcalProperty(marshaller, "");
		assertXMLEqual(expected, actual);
	} 
                            #location 6 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                             @Test
	public void writeXml_null() {
		DateTimePropertyImpl prop = new DateTimePropertyImpl(null);
		assertWriteXml("", prop, marshaller);
	}