[BUGFIX beta] Remove old reference to invoker.

As of 6a8137902da56b8b1060fcd0bdb748d5e7ae0092 we no longer use a custom
`invoker`, and the logic was moved into `registerObserver` directly.
Merge pull request #4686 from rjackson/fix-removed-invoker

[BUGFIX beta] Remove old reference to invoker.