[Bug Report] When the cascade selector dynamically changes 'options' and 'value', the 'handleExpand' method of the cascade panel throw an error: "Cannot read property 'level' of null" #18369
Description
Element UI version
2.13.0
Component Name
cascader
OS/Browsers version
Win10 x64 / Chrome 78.0.3904.97
Vue version
2.6.6
Reproduction Link
https://jsfiddle.net/WalkingLianshou/wb6sat3h/31/
Steps to reproduce
Switching the console directly to the selector will throw an error, and when switching again, the default selection and expansion of the cascade selector will be wrong.
直接对选择器进行切换控制台就会抛错，并且再次重新切换时，级联选择器的默认选中及展开出现错误
What is Expected?
After the selector is switched, the cascading selector is not thrown first, and the second is selected correctly. After switching back again, the style and display are normal.
选择器切换后，级联选择器首先不抛错，其次正确的选中，再次切换回来后，样式及展示正常;
What is actually happening?
After switching the selector directly, throw "Cannot read property 'level' of null" and switch back to the style and default expansion error again.
直接切换选择器后抛错"Cannot read property 'level' of null"，再次切换回来样式及默认展开出现错误;