Fixed: presentPopoverFromRect:inView: causes SIGABRT

Steps to reproduce:
1. Run TokenFieldExamle in iPad
2. Touches the search button without type anything
3. App crashed,see the console
Commit 'forbit
auto-searching'(SHA:81311ee01656fc44202acbaa0c96e238bffcaac2 ) causes
this bug.
This commit fixed it.