How to trouble-shoot 'critical error' while merging duplicates?
I'll dig deeper in another post, but as far as this problem was concerned, I used print and var_dump to trace the problem to 

public static function getReferencesToTable
.
.
.
$links = $daoClassName::getReferenceColumns();


'crashes' because one of the classes was NULL. 

While I work on WHY, I got around the crash with a simple:

if ( $daoClassName != NULL)


wrapper.