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During migration you should check your old code for places where the AjaxRequestCycle (now an Optional<AjaxRequestTarget>) is compared with null:

if (cycle.find(AjaxRequestTarget.class) == null) {
  // this is *never* executed since #find() always returns an Optional
}

if (cycle.find(AjaxRequestTarget.class) != null) {
  // this is *always* executed since #find() always returns an Optional
}