Avoid force-unwrapping, which was causing a crash. Add an `assertionFailure` so we can catch this issue in the act — we need to know why an item would not be a Node. Fix https://github.com/Ranchero-Software/NetNewsWire/issues/3824

Mac/MainWindow/Sidebar/SidebarViewController.swift

@@ -371,7 +371,10 @@ protocol SidebarDelegate: AnyObject {
 	}
 
 	func outlineView(_ outlineView: NSOutlineView, isGroupItem item: Any) -> Bool {
-		let node = item as! Node
+		guard let node = item as? Node else {
+			assertionFailure("Expected item to be a Node.")
+			return false
+		}
 		return node.isGroupItem
 	}