[bug report] form error

### Element UI version
2.4.6

### OS/Browsers version
windows

### Vue version
2.5.9

### Reproduction Link
https://elementui.github.io/issue-generator/#/zh-CN

### Steps to reproduce
TypeError: Cannot read properties of undefined (reading '__resizeListeners__')
at addResizeListener (resize-event.js:42)
at VueComponent.mounted (scrollbar.js:375)
at invokeWithErrorHandling (vue.esm.js:1871)
at callHook (vue.esm.js:4238)
at Object.insert (vue.esm.js:3164)
at invokeInsertHook (vue.esm.js:6391)
at VueComponent.patch [as __patch__] (vue.esm.js:6610)
at VueComponent.Vue._update (vue.esm.js:3966)
at VueComponent.updateComponent (vue.esm.js:4084)
at Watcher.get (vue.esm.js:4498)

### What is Expected?
TypeError: Cannot read properties of undefined (reading '__resizeListeners__')
at addResizeListener (resize-event.js:42)
at VueComponent.mounted (scrollbar.js:375)
at invokeWithErrorHandling (vue.esm.js:1871)
at callHook (vue.esm.js:4238)
at Object.insert (vue.esm.js:3164)
at invokeInsertHook (vue.esm.js:6391)
at VueComponent.patch [as __patch__] (vue.esm.js:6610)
at VueComponent.Vue._update (vue.esm.js:3966)
at VueComponent.updateComponent (vue.esm.js:4084)
at Watcher.get (vue.esm.js:4498)

### What is actually happening?
TypeError: Cannot read properties of undefined (reading '__resizeListeners__')
at addResizeListener (resize-event.js:42)
at VueComponent.mounted (scrollbar.js:375)
at invokeWithErrorHandling (vue.esm.js:1871)
at callHook (vue.esm.js:4238)
at Object.insert (vue.esm.js:3164)
at invokeInsertHook (vue.esm.js:6391)
at VueComponent.patch [as __patch__] (vue.esm.js:6610)
at VueComponent.Vue._update (vue.esm.js:3966)
at VueComponent.updateComponent (vue.esm.js:4084)
at Watcher.get (vue.esm.js:4498)