Sorry for bad news on Friday but it looks like the newest build ( 2021.4.5156) is broken in couple of places.
First problem: When initial rendering of events is done, the "Cannot read properties of null (reading 'rendered')" error is thrown. This always happens twice with initial load.
Second problem: When dynamicEventRendering is set to 'Progressive' now, when scrolling the same error as above mentioned appears.
Besides that, in SchedulerBeforeTimeHeaderRenderArgs - areas property is not exposed in header property. It's still present in your documentation.