UIActivityViewController iOS8 iPad Crash处理

在iPhone 上程序一切正常，但独在iPad iOS8的系统上,UIActivityViewController会崩溃，经查阅资料：

On iPad the activity view controller will be displayed as a popover using the newUIPopoverPresentationController,
it requires that you specify an anchor point for the presentation of the popover using one of the three following properties:

barButtonItem

sourceView

sourceRect
In order to specify the anchor point you will need to obtain a reference to the UIActivityController's UIPopoverPresentationController and set one of the properties as follows:activityViewController.popoverPresentationController.sourceView = parentView;
// 或者
activityViewController.popoverPresentationController.barButtonItem = _actionButton;
// 或者SourceRect(不再举例)

资料地址:  http://stackoverflow.com/questions/25644054/uiactivityviewcontroller-crashing-on-ios8-ipads