This is a breakchange of JS API. From JS API 4.27, The Popup widget loading is deferred until the view is ready and finished updating, and will only be loaded if View.popupEnabled is set to true (which is the default), when View.openPopup() is called, or when some widgets need the popup, such as Search. That is to say, in API 4.26, after View is initialized, View.popup is a Popup instance, but in API 4.27, after View is initialized, View.popup is just an ordinary object. Only after calling the View.openPopup() method, View.popup will become a Popup instance. See details here - https://developers.arcgis.com/javascript/latest/breaking-changes/#427
I think you can call View.openPopup(), when this method is resolved, you can call view.popup.fetchFeatures().
