Fixes bug where Slate crashes if window hints is triggered in certain contexts (Emacs.app or Calendar.app visible)

NOTE: This bug is not reproducible if .slate has <code>config windowHintsIgnoreHiddenWindows false</code>
Bug previously raised in this issue:
https://github.com/jigish/slate/issues/303

Patch is simply a one line change to initialize local variable in AccessibilityWrapper.m:getTitle:window which subsequently fixes the bug.