修复Cron可视化查看数据清空问题；

xxl-job-admin/src/main/resources/static/plugins/cronGen/cronGen.js

     var generate = function () {
 
         var activeTab = $("ul#CronGenTabs li.active a").prop("id");
+        if (activeTab == undefined) {
+            return;
+        }
         var results = "";
         switch (activeTab) {
             case "SecondlyTab":

xxl-job-admin/src/main/resources/static/plugins/cronGen/cronGen_en.js

     var generate = function () {
 
         var activeTab = $("ul#CronGenTabs li.active a").prop("id");
+        if (activeTab == undefined) {
+            return;
+        }
         var results = "";
         switch (activeTab) {
             case "SecondlyTab":