Balance sidebar `Deref` cycle check with main content
The `Deref` cycle checks added as part of #80653 were "unbalanced" in the sense
that the main content code path checks for cycles _before_ descending, while the
sidebar checks _after_. Checking _before_ is correct, so this changes the
sidebar path to match the main content path.
Fixes#81395
r? ```@GuillaumeGomez```