TTL `getSession` function may return unexpected result

[YangKeao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

The function getSession returns a session and an error. It's expected to return either session or an error. However, it's possible that both of them are nil, because the Close() function may overwrite the err return value.

2. What did you expect to see? (Required)

At least one of session and err are not nil.

3. What did you see instead (Required)

Both of them are nil if the session is error.

[YangKeao]

I believe it's a super corner case and will rarely occur in real world.