Fix DatePicker crash on iPad

[Tug]

I noticed the following error while using the universal design system you've built and trying to open the date picker on iPad:

Your application has presented a UIAlertController (<UIAlertController: 0x7fa4e56dda00>) of style
UIAlertControllerStyleActionSheet from UIViewController (<UIViewController: 0x7fa500c59390>). The 
modalPresentationStyle of a UIAlertController with this style is UIModalPresentationPopover. You must provide location 
information for this popover through the alert controller's popoverPresentationController. You must provide either a 
sourceView and sourceRect or a barButtonItem.  If this information is not known when you present the alert controller, you
may provide it in the UIPopoverPresentationControllerDelegate method -prepareForPopoverPresentation.

This PR is an attempt to fix it.
Disclaimer: my knowledge of iOS is very limited.

How to reproduce:

• Open the "iPad (10th generation)" Simulator
• Load the master branch of this project
• yarn; cd example; rm -rf ios; yarn ios
• Wait for the example app to load on the iPad sim
• Click on the "Open picker" button, you should see the error in the console

Testing Instructions

• Apply this patch
• yarn ios
• Check that the error is gone and that the date picker looks decent on iPad