PopupManager: fix z-index cannot be rewritten at first using (#15738)

* PopupManager: fix z-index value cannot be rewritten at initial render (#15074)

* fix issue

* remove package.lock

src/utils/popup/popup-manager.js

@@ -3,7 +3,7 @@ import { addClass, removeClass } from 'element-ui/src/utils/dom';
 
 let hasModal = false;
 let hasInitZIndex = false;
-let zIndex = 2000;
+let zIndex;
 
 const getModal = function() {
   if (Vue.prototype.$isServer) return;
@@ -155,7 +155,7 @@ Object.defineProperty(PopupManager, 'zIndex', {
   configurable: true,
   get() {
     if (!hasInitZIndex) {
-      zIndex = (Vue.prototype.$ELEMENT || {}).zIndex || zIndex;
+      zIndex = zIndex || (Vue.prototype.$ELEMENT || {}).zIndex || 2000;
       hasInitZIndex = true;
     }
     return zIndex;