Issue
Trying to manually run Joomla Smart Search Indexer immediately fails with following error:
Indexer Running
undefined
null
Resolution
As it often the case with similar Joomla errors the issue was caused by a misbehaving Joomla plugin. To find the offending plugin I had to disable all content type plugins one by one (clearing browser cache each time), until the Smart Search Indexer run successfully. In this case the issue was cased by AddThis - Bookmark and Sharing Tool plugin.
Removing the above plugin resolved the issue.
February 2017
Joomla 3.6.5
AddThis Plugin
