Fixed UIPopoverPresentationController should have a non-nil sourceView or barButtonItem crash

Setting shareButton as the barButtonItem of UIActivityViewController's
popoverPresentationController to address the following crash:

Terminating app due to uncaught exception 'NSGenericException',
reason: 'UIPopoverPresentationController
(<_UIAlertControllerActionSheetRegularPresentationController:
0x7f89f85c53a0>) should have a non-nil sourceView or barButtonItem set
before the presentation occurs.'