最后页面访问Controller报500，请问怎么解决？

Question

项目启动正常，hello也能访问，dao测试也成功，但是最后页面访问Controller报500，查看路径也没错控制台报错2019-03-20 20:01:03.650 ERROR 11652 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[.[dispatcherServlet]      : Servlet.service() for servlet [dispatcherServlet] in context with path [/demo] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root causejava.lang.NullPointerException: null at com.imooc.demo.web.AreaController.getAreaById(AreaController.java:52) ~[classes/:na]