[core] Fix the risk of iterator invalidation issue. (#20989)
[core] Fix the risk of iterator invalidation issue. (#20989)

We erase the elements from object_id_refs_ in the method `RemoveLocalReferenceInternal()` which may cause iterator invalidation issue.

Note that, normally flatmap will not trigger any iterator invalidation except triggering `rehash()`. But in this case, we may remove other elements(not only the current iterator), so there is still a risk of it.
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