Joomla 2.x Smart Indexer fails with "undefined , undefined, null"

Recently ran into this issue on this very site so I did some digging.

This is apparently a issue with one of the plugins you have installed. On this site we use codehighliter by Union D and apparently this doesn't handle Joomlas Smart Indexer indexing correct.
To fix this do the following

1. Find the plugin that is causing the issue - in my case I checked the error_log on the webserver. Quite apparent in the log as it says "Call do undefined method..." and in there you will also find "referer: XxXxX?option=com_finder&view=indexer&tmpl=component
2. Edit the file where the issue seem to be - see 1.
3. Do a search for " onContentPrepare "
4. Insert the following just below it

// Don't run this plugin when the content is being indexed
if ($context == 'com_finder.indexer')
{
    return true;
}

This should fix the issue - it did for me!