Fixed a bug

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.'
Fixed UIPopoverPresentationController crash

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

Before presentation, check if
controller.popoverPresentationController.barButtonItem is null; it is
null, assign shareButton to it.
Merge pull request #30 from yichizhang/master

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