I found the issues causing that error ... a very minor thing the way a variable is initialized as a string and not an array.
It is
PHP 7.1 related: (I have PHP 7.1.1, and I saw a similar error on a different line number)
php.net Wrote:Note: As of PHP 7.1.0, applying the empty index operator on a string throws a fatal error. Formerly, the string was silently converted to an array.
I'll commit the change in a moment.
Change committed
to SVN.