[Bug Report] 在el-table组件的template slot-scope="scope"作用域内使用el-cascader的getCheckedNodes失败

### Element UI version
2.13.2

### OS/Browsers version
windows10

### Vue version
2.6.10

### Reproduction Link
https://github.com/ElemeFE/element/issues/19489#event-3371581933

### Steps to reproduce
在el-table组件的template slot-scope="scope"作用域内使用el-cascader的getCheckedNodes失败，
ref如果是动态命名时，返回的内容是空数组。
将ref写死固定后，首次返回空数组，之后每次都返回node值

为了排除，将el-cascader写在el-table外，调用一切正常。但是放入el-table内部，就会出现问题

vue-----------------------------------------------------------

{{scope.row.title}}










{{scope.row.orgIdName}}









{{scope.row.region}}










{{scope.row.takeTime}}









编辑
保存
删除

js------------------------------------------------------------------------------------------------

updateCasChange(val, index, num) {
let nodeObj = this.$refs['orgIdindex' num index].getCheckedNodes();
console.log(val.orgId, "222-updateType")
console.log(index, "333-updateType")
console.log(num, "444-updateType")
console.log(nodeObj, "nodeObj")
},

### What is Expected?
Please tell me the solution

### What is actually happening?
Please tell me the solution