Ios filepicker cancel

[Orillion360]

Fixes exception thrown when you open the file picker for the second time after clicking cancel the first time.

[jfversluis]

This was added manually to the latest commit (https://github.com/Studyxnet/FilePicker-Plugin-for-Xamarin-and-Windows/commit/db159f3c0578cbfff425c89dc602dd44349b5b88) hence this can be closed