I get this error when I fast push and pop pages (using back button) many times into navigation stack (tested on iPad Air iOS 8.4).
I think that there is a problem with popping pages using native back button. I'm awaiting all my calls to PushAsync() method and always check IsBusy flag for each page which I want to navigate from. Unfortunately there is no possibility to do the same when user pops a page using back button on native nav bar. All my page instances are singletons. I'm using XF 220.127.116.1176.
I read this thread https://forums.xamarin.com/discussion/34515/inavigation-popmodalasync-does-not-set-modal-pages-parent-property-to-null but my error still occures after upgrade.