Avoid errors with undefined populator, selector

For reasons I could not determine, and which disappear whenever you add a
debugger to the mix (that is, it is a race condition), The populator and helper
members are sometimes undefined causing errors. In addition, this code makes
sure we don't get the entire hierarchy if more than one parent matches
the selector, for we most certainly don't want more than one popup.

Fixes #408.
Sync with coffee
Merge pull request #3 from oerpub/oerpub-ticket408

Avoid errors with undefined populator, selector