In \XF\Service\PusherTrait there is a protected $receiever; definition that is spelled incorrectly. However, everything referencing it is spelled correctly causing issues with type hinting in phpStorm
Thank you for reporting this issue, it has now been resolved. We are aiming to include any changes that have been made in a future XF release (2.2.0 Beta 2).
Change log:
Fix typo in PusherTrait's class property definition