Fix NullPointer Exception

src/main/java/cn/celess/blog/configuration/filter/VisitorRecord.java

@@ -21,7 +21,11 @@ public class VisitorRecord implements HandlerInterceptor {
     @Override
     public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
         HttpSession session = request.getSession();
+
         HashMap<String, Integer> visitDetail = (HashMap<String, Integer>) session.getAttribute("visitDetail");
+        if (visitDetail == null) {
+            return true;
+        }
         // 获取访问次数
         Integer count = visitDetail.get(RequestUtil.getCompleteUrlAndMethod(request));
         // 自增