[bug report] calculation bug caused by no reset of cascade select panel activepath

### Element UI version
2.13.2

### OS/Browsers version
mac os 10.14

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/chenzhq/pen/WNQPxmQ?editors=1111

### Steps to reproduce
1. 点击cascade select组件，选择任一节点，只需要修改activePath即可；
2. 关闭select panel；
3. 点击【修改】按钮，修改options；

### What is Expected?
重置value，重置activePath

### What is actually happening?
console里会报错 初步定位是由于acitvePath未重置，导致计算node时出错。