fix[Dashboard]: fixed debounce bug (#2586) (#2597)

To fixed https://github.com/PanJiaChen/vue-element-admin/issues/2586

src/views/dashboard/admin/components/mixins/resize.js

@@ -3,10 +3,16 @@ import { debounce } from '@/utils'
 export default {
   data() {
     return {
-      $_sidebarElm: null
+      $_sidebarElm: null,
+      $_resizeHandler: null
     }
   },
   mounted() {
+    this.$_resizeHandler = debounce(() => {
+      if (this.chart) {
+        this.chart.resize()
+      }
+    }, 100)
     this.$_initResizeEvent()
     this.$_initSidebarResizeEvent()
   },
@@ -27,13 +33,6 @@ export default {
   methods: {
     // use $_ for mixins properties
     // https://vuejs.org/v2/style-guide/index.html#Private-property-names-essential
-    $_resizeHandler() {
-      return debounce(() => {
-        if (this.chart) {
-          this.chart.resize()
-        }
-      }, 100)()
-    },
     $_initResizeEvent() {
       window.addEventListener('resize', this.$_resizeHandler)
     },