I think I have found a bug in your datagrid. If you open a datagrid's filter pop-up, navigate away from the page, then go back to the page, the filter pop-up will still be open. If you attempt to close it, by way of the "X", the app will crash. In debug mode, a null reference error will occur. If navigating away from the grid with the filter closed, no crash will happen. This happens both in a tabbed view, switching from one tab to to another and back, and in the xamarin shell navigation. This happens in both my app and the example app you provide.
Is there a way to close the filter pop-up upon un-focus or switching views or something?