[bug report] error is reported after the options are changed after the cascader component is selected

### Element UI version
2.13.0

### OS/Browsers version
Chrome

### Vue version
2.6.10

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

### Steps to reproduce
级联选择器 Cascader，
更新 options 属性，
选择某个子节点，
更新options 属性，
报错。
Error in callback for watcher "options": "TypeError: Cannot read property 'level' of null"

found in

---> at packages/cascader-panel/src/cascader-panel.vue
at packages/cascader/src/cascader.vue

--------------------------------------------------------------------
vue.runtime.esm.js:1888 TypeError: Cannot read property 'level' of null
at VueComponent.handleExpand (cascader-panel.js:1368)
at cascader-panel.js:1312
at Array.forEach ()
at VueComponent.expandNodes (cascader-panel.js:1311)
at VueComponent.syncActivePath (cascader-panel.js:1297)
at VueComponent.syncMenuState (cascader-panel.js:1270)
at VueComponent.initStore (cascader-panel.js:1254)
at VueComponent.handler (cascader-panel.js:1219)
at Watcher.run (vue.runtime.esm.js:4562)
at flushSchedulerQueue (vue.runtime.esm.js:4304)
--------------------------------------------------------------------
vue.runtime.esm.js:619 [Vue warn]: Error in callback for watcher "value": "TypeError: Cannot read property 'level' of null"

found in

---> at packages/cascader-panel/src/cascader-panel.vue
at packages/cascader/src/cascader.vue
--------------------------------------------------------------------
vue.runtime.esm.js:1888 TypeError: Cannot read property 'level' of null
at VueComponent.handleExpand (cascader-panel.js:1368)
at cascader-panel.js:1312
at Array.forEach ()
at VueComponent.expandNodes (cascader-panel.js:1311)
at VueComponent.syncActivePath (cascader-panel.js:1297)
at VueComponent.syncMenuState (cascader-panel.js:1270)
at VueComponent.syncCheckedValue (cascader-panel.js:1263)
at VueComponent.value (cascader-panel.js:1225)
at Watcher.run (vue.runtime.esm.js:4562)
at flushSchedulerQueue (vue.runtime.esm.js:4304)

### What is Expected?
期待 可一动态更新 options 属性 使 Cascader 可以动态更新。

### What is actually happening?
实际可以动态更新，可是会报错，期待无报错。