fix(components): [table] getColumnIndex is undefined when unmount

[Liao-js]

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[btea]

Can you provide a use case scenario where the error occurs?

[Liao-js]

Can you provide a use case scenario where the error occurs?

last night, the PR developer raised a question in my gihub, saying that using el-table in vuedraggable would report an error, causing the execution to fail.I didn’t expect that he submitted a PR this morning. Let me close this PR.

[Liao-js]

when I saw his question, I also felt that my code was not rigorous enough. Just mentioned this PR