[Improvement][Fix-5769][UI]When we try to delete the existing dag, the console in web browser would shows exception (#5770)

* fix bug

* cache the this variable

* Avoid self name

* fix code style compile error

dolphinscheduler-ui/src/js/conf/home/pages/dag/_source/dag.js

@@ -110,6 +110,7 @@ Dag.prototype.toolbarEvent = function ({ item, code, is }) {
  * Echo data display
  */
 Dag.prototype.backfill = function (arg) {
+  const that = this
   if (arg) {
     const marginX = 100
     const g = new dagre.graphlib.Graph()
@@ -144,7 +145,7 @@ Dag.prototype.backfill = function (arg) {
         instance: this.instance,
         options: {
           onRemoveNodes ($id) {
-            this.dag.removeEventModelById($id)
+            that.dag.removeEventModelById($id)
           }
         }
       })
@@ -167,7 +168,7 @@ Dag.prototype.backfill = function (arg) {
         instance: this.instance,
         options: {
           onRemoveNodes ($id) {
-            this.dag.removeEventModelById($id)
+            that.dag.removeEventModelById($id)
           }
         }
       })