Error rapidly clicking "Mark as Done" (#6951)

Fixes #6922

Author: alexr00

src/notifications/notificationItem.ts

@@ -20,6 +20,6 @@ export interface NotificationTreeItem {
 	readonly kind: 'notification';
 }
 
-export function isNotificationTreeItem(item: any): item is NotificationTreeItem {
-	return item.kind === 'notification';
+export function isNotificationTreeItem(item: unknown): item is NotificationTreeItem {
+	return !!item && (item as Partial<NotificationTreeItem>).kind === 'notification';
 }