Can macOS Safari App Extensions be purged for example in low memory situations?

I'm building a safari app extension with objective c. I'm using the default Xcode project. I've successfully added a NSView to the popover window. When I drag a NSTableView into the popover window, it appears in the XIB, but I can't find how to connect it to the SafariExtensionViewController. I tried adding a custom ViewController to the XIB and making the tableview its view which seems to be ok. How should I proceed from here, or is their another approach?

I'm trying to get the row index of a NSTableRowView that has experienced a mouseEnter event. It should be easy with rowAtPoint using NSEvent.locationInWindow. I can't get it to work. The NSTableView lies inside a NSStackView. Of course the NSStackView adjusts the visual size of the NSTableView etc. Is NSStackView interfering? If so, how should I adjust my call to rowAtPoint?

As far as I can determine, there is no way to save the data in my Safari App Extension when Safari terminates. It seems I have to save the data every time the page changes. That's wasteful. Does anyone know of a work around?