With the CMS 4.12 update functionality was altered to utilise an
Extension to obtain the CMS Edit link for a page, rather than having
SiteTree do it internally. Unfortunately the default return case for
`extend` (see Extensible) is an _empty_ array. This leave code
potentially referencing an array offset that doesn't exist ([0]). PHP 8
is less forgiving that it's predecessors on this kind of behaviour. We
should check that the responses from extensions exist before trying to
reference them.
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