If we have a sort_on parameter in base_query,
  use it instead of getObjPositionInParent to display folder content.
Modified pagination links selector to use only ".listingBar" instead of
"div.listingBar". The batchnavigation.pt template can be customized in a
theme and use a different structure, "ul.listingBar" for example.
Preparing release 2.4.19
Back to development: 2.4.20
fix tests
Filter out references that are None.

This may happen when the reference_catalog has links from a source to
a no longer existing target.
Nuke trailing white space.
pep8
Fix test for contents of base widget html.