Fix for "Sheet can not be presented..." crash

Fixes https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/371. I was not able to repro this issue, however given the error, I think it is safe to assume the toolbar is not in the window and displaying the actionsheet with it in that state is bad news. :grinning: 

There may be some relation to this bug: https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/372 but for now, this fix should alleviate some of the crashers.

/cc @sendhil 

P.S. it looks like the ```Podfile.lock``` file was out of date too...updated to correct commit hash.