Also posted here:
Looks like there was a regression sometime between Xamarin.Forms 126.96.36.1999982 and version 188.8.131.522540 with the DataGrid. After a DataGrid becomes visible on a page (or just on a page that becomes visible, even if the DataGrid itself is set to IsVisible = false), a NullReferenceException occurs if you navigate away or switch tabs in a RadTabView. Seems to only happen on iOS, and started happening when I upgraded from Xamarin.Forms from 184.108.40.2069982 to version 220.127.116.112540. Possibly a Xamarin.Forms bug and not a Telerik bug.
In case anyone else has the same issue, the workaround for me is just to go back to Xamarin.Forms 18.104.22.1689982 (not sure if the issue occurs in the other 3.2.* version in between).